You can often find a great deal, especially at short notice, if you're willing to make reservations without knowing your specific hotel before you agree to buy. Two companies that offer deals of this kind are Hotwire and Priceline.

Hotwire works with major hotel chains to sell unsold rooms at discounted rates. Because Hotwire has a very detailed hotel rating system, you can get a sense of how good the deal is by comparing the price Hotwire offers you with the lowest published rate you've found for a similar quality hotel in the same location.

Hotwire will offer you a complete price, including taxes and fees, though it won't show the exact breakdown between those taxes and fees. Also, you won't know your hotel name or exact location until you agree to book the room. Once you see a price you like, be sure to book it quickly, as rates are subject to availability.

If you want to "name your own price" for a hotel room, Priceline is another type of bidding site that allows you—and not the hotel—to set the price. To use Priceline, you also have to be willing to make reservations without knowing your hotel or its exact location. But also like Hotwire, Priceline rates its hotels from one to five stars, and you can get a good idea of the type of hotel you'll be bidding on.

If Priceline accepts your bid, you're locked into the rate. To make sure you don't overbid, you should check the lowest published rates first, and bid about 20 to 30 percent lower. You'll know within a few minutes if Priceline has accepted your bid; if not, you can re-bid by changing one of your choices, or go back to Hotwire and book your room there. Your total cost on Priceline is your accepted bid plus hotel taxes and a processing charge per ticket, so be sure you are comparing the total amount to see which site offers you a better deal.

With both Hotwire and Priceline, be very careful when booking because your reservations are entirely nonrefundable and nonchangeable. Also, note that you won't earn hotel loyalty points for your stay.