I'm already assuming that you're looking only at hotels rated three-star/moderate and below. These tips will help you whittle the rates down a good 10 to 40 percent below the asking price.
Before we begin, something that is not so much a tip as a point to remember: by and large, in the U.S. you're charged by the room. In Europe you're charged by the head count.
This is why, as a frugal Assistant Scoutmaster who doesn't believe tent camping was invented with cold rainy nights in mind, I can take a group of Boy Scouts and cram 16 of them into one cheap motel room for $39.95 in the USA. But when I took them to Europe in the summer of 2000, I had to pay for lodgings on a per-scout basis (though I did usually get a "bulk discount").
What I mean is, while a four-bedded "quad" room will be cheaper than renting two double rooms, you are not going to convince the hotelier to charge even less for your willingness to squeeze four people into one double room. It just doesn't work that way over there.
Cardinal law of European hotel rooms: you pay more to have a private bath in the room. If you don't mind sharing the common bathroom down the hall, you can easily save 25%–50%. Just like that. Just for being willing to carry your bathroom gear down the corridor, and for occasionally have to wait a bit for someone else to vacate the facilities.
In most places (aside from hostels), you usually share the hall bath with, at most, four or five other rooms, more often two or three. Frequently you get it all to yourself; there was just no space to install a bathroom in your bedroom, so you have to use a private one out in the corridor.
What's more, in the vast majority of hotels even the bathless rooms come equipped with at least a sink, they just lack the shower/tub or a toilet (and some even have the shower as well, just not the commode). That means for simple ablutions and the hand-washing of your clothes you're still all set. And don't worry, you can still hear nature calling from just down the hall.
I don't mean don't eat breakfast, just don't eat it at your hotel. If you have the option of opting out of breakfast and getting something knocked off your hotel bill, do so.
Usually hotel a breakfast costs anywhere from $5 to $15 per person, and—except in British and Irish B&Bs, some farm stays, or a Scandinavian smorgasbord—normally consists of croissants and/or rolls, maybe some packaged jams, coffee or tea, and juice.
Six main hotel rip-offs
Hotel breakfasts aren't the only rip-offs at the inn. Here's the skinny on some perfectly legal hotel scams:
• the minibar
• the telephone
• the parking garage
• the breakfast
• the laundry service
• the taxesYou can get a cappuccino and croissant from the corner cafe for $4 or less. Plus, if you patronize the local bar, you get the chance to rub elbows at the bar with locals on their way to work rather than share a hotel breakfast in a room filled with other tourists. Only on very rare occasions and in the very cheapest hotels do they charge you as little for breakfast as the local cafe would.
If, however, your hotel insists that breakfast is included in the rate and you cannot opt out, then you have carte blanche to bring your daypack down to breakfast with you and load it up with enough extra food to make at least a decent mid-morning snack if not a light picnic lunch out of it. 
Rock yourself to sleep on an overnight train for $0 to $30
OK, first the $30 version. If you've got to traverse vast distances on your trip—train rides of more than six or seven hours—you can either (1) waste basically an entire day of your vacation getting from point a to point b, or (2) book a sleeping berth (couchette) for $20 to $30 on an overnight train, sleep while you travel, and end up both paying far less for accommodations that night than you would at a hotel and getting where you're going without blowing off a whole day.
Sure, you miss out on the pretty scenery, plus truth be told train bunks provide far from a comfortable and sound night's sleep, but all in all it's not a bad deal. Always book a second class couchette, which will usually land you one of six narrow bunks in a tiny compartment, because you really aren't going to get much more out of the pricier options anyway.
Now, about that $0 option. This one is getting harder and harder to finagle. I blame "progress." Old style European trains were done up with an all-compartment configuration, each car made up of ten or so small rooms, each with its own sliding door and picture window. Each seats six people each in two facing rows, all connected by a corridor running along the edge of the car.
This civilized arrangement, where you often get to know the other passengers in your compartment and maybe end up sharing a picnic or at least learning something, is being slowly phased out in Western Europe in favor of the soulless, straight-through car configuration familiar to any American commuter, where two-seat-wide rows line either side of a central aisle, everyone facing the same way (or, sometimes, half the car facing forwards, the other backwards).
If you're lucky and end up on of the old-style trains, the seat bottoms in those compartments will often pull out into the middle of the room while the seat backs collapse down. Pull out two opposing seats, and you have one continuous, padded flat surface spanning the compartment. Pull out all six seats, and you've got a romper room of sorts, a roughly kind-sized bed that take up the entire compartment.
Just slide the compartment door shut, close the shades, turn out the lights, and at least pretend to be asleep so that, hopefully, new people boarding the train will refrain from disturbing you and find some other compartment to sit in.
Of course, the danger is that they will not pass you by, the lights will flick back on, and you will spend the night sitting up and miserably wreathed in cigarette smoke while your new companions chatter to each other loudly.
Crowd the clan into one room
If you don't need the privacy, don't rent a separate room for the kids, as it will cost twice as much. An extra bed in your room will cost, at most, 35 percent more. Cots and cribs cost even less (sometimes nothing, if the kids are young enough).
Triple or quads (rooms with three or four regular beds already in them) are more expensive than a double room, but less expensive than one double plus one single, or two doubles, respectively.
You get my drift. If you find you do need a bit more privacy on occasion, ask if a hotel has family suites, where two separate rooms share a common door to the hall, or there are two bedrooms within the guest quarters.
Snuggle up: Double beds cost less than two singles
This is a rule of thumb slowly fading from the European lodging scene, but still in some hotels you will find that a room with a "double bed" (double or queen) will cost a bit less than one with two single beds, mainly because they have to wash only one set of sheets.
In several European languages double beds are called the local lingo's word for "matrimonial;" just call it "one big bed" to get your point across. In point of fact, a "double" bed often ends up being two singles pushed together with a queen-sized sheet stretched across both.
Bonus hint: if the seam in the middle bothers you or the gap begins to widen as the cots underneath slowly slide away from one another under your weight, unmake the bed and rotate the pair of mattresses 90 degrees to that the seam is now horizontal.
Find out if taxes are included in the hotel quote. In much of Europe, this is not an issue as the country's VAT—the Value Added Tax, sort of like a national sales tax—is automatically folded into the sticker price, as it were (the same is true of almost all purchases in Europe).
However, there are a few countries where they might set the room rates before taxes—or there are special hotel taxes (taxe de sojour) that are applied above and beyond VAT—and tack on the extra when it comes time to pay the bill. There's nothing illegal about this at all, just a bit sneaky, and as with so many other hidden or inflated charges, the pricier the hotel is, the more likely it is to leave the tax out of its posted rates.
Look out for extra hotel taxes especially in Spain (7%) and France (it varies depending on the hotel's star rating). It happens much less frequently, but with rather more dramatic results in Britain (where the tax is a whopping 20%) and the more expensive properties in the Czech Republic (where you may find a 22% headache waiting for you at the end of your stay).
Be cheap: Ask for the least expensive room
Yeah, seems pretty obvious, but you'd be surprised how often people overpay for one room when another in the same hotel costs less just because it's slightly smaller, or doesn't have "the view" (of the lake or sea or cathedral or whatever), or isn't one of the recently renovated rooms. If they quote you one price, always ask "Do you have one that is cheaper?" Which brings me to:
We Yanks have earned something of a reputation for constantly asking for cheaper rates than those quoted or posted-or at least European hoteliers complain that we do so, and tend to chastise me, the travel writer, for continuing to recommend this tactic, even though I see locals doing it as much or more often than Americans do...but I digress.
If it's the dead of winter and the hotel is empty, try to haggle the price down a bit, maybe 10%–30%. Don't bother trying during major holidays, at the height of the high season, or when the place is booked solid.
Pay in cold hard cash, baby
If you pay be credit card, the hotelier will charge you the posted, official rate and then has to tithe a certain percentage of that to the credit card company for the sake of your convenience. If you pay cash, he gets to keep every last Euro of it, so he's likely to knock a few bucks off the price to sweeten the deal.
Always ask, after the price is quoted, "Is that the price if I pay with a credit card? What if I pay with cash?" Often the rate will magically come down. Also—though you didn't hear this from me—if you pay cash, it leaves the hotel free to underreport their income to the tax man.
The room may cost €50, and they'll claim they sold it to you for maybe €30, and then you have to pay just €40 in actual currency because, for them, that €10 is pure profit.
Shack up in Verona instead of Venice, Haarlem instead of Amsterdam, or Potsdam instead of Berlin. Not every major European city has a secondary city of significant interest in its own right that's close enough for this to work, but some do.
This option is a bit of a triple-edged sword, if such a metaphor is possible. It is almost always cheaper to stay in a smaller satellite or nearby city than it is to stay in the major/popular one, especially during high season.
However, even though you are saving money, you are stuck staying in somewhere other than the big city you came to see, plus you've got to factor in the price of the train you take every day for that 20-minute ride into the big city. Even so, you do get a chance this way to experience two cities, one smaller and far less touristed than the main one, which only enriches your overall experience.
This option is not for everyone, certainly not if it's your visit to the major city and/or you have only a limited time to see it, but it can be a welcome option if the main city is booked solid in high season or during some trade fair.
Shop around
Call a number of hotels from the train station when you arrive. If the city doesn't appear to be full (if everyone has vacancies), don't settle for the first place with an empty bed. Find the perfect balance between where you want to stay (a sumptuous suite with a private pool overlooking the cathedral in the center of town) and what you want to pay (not enough to afford that).
Find out what the lodging market is like in town on that day, pick your ideal hotel, and then bargain. If you play it right, you can end up netting yourself twice the room at half the cost than the bozo who was on the train next to you, blindly follows his guidebook's advice, and grabs the first room he finds at the first price quoted him.
Look into lodging alternatives
Hostels, farm stays, rental rooms, campgrounds, B&B's, monasteries...there are so many of these budget options (more than two dozen) I needed to create a whole separate section on this Website just to fit them all in.
