Priceline is just awesome for hotels. With Priceline, it's not uncommon to get a $40 room in a 3-star hotel. Here's a tip to getting a great rate: always low ball your first bid. You will get the option to increase it later, so why not start low? Keep in mind that prices are often lower during the off-season and on weekdays. Shoot for a nice hotel while you're at it, since prices for higher quality hotels can be about the same as for lower quality hotels. Expect to save at least 50%.
Bundle with your Airfare:
Popular search engines such as Travelocity and Expedia offer package deals when you purchase both airfare and lodging. These services are nice because, unlike Priceline, you get to choose the specific hotel. But you don't need to bundle to save money with these sites, so check them out anyway.
Hostels:
If you are traveling alone, youth hostels can be an inexpensive option, usually around $20/night. If traveling with others, you might save money staying in a hotel, since hostels charge on a per-person basis. For the very frugal, some hostels will let you stay for free in exchange for work.
Ask about Discounts:
Are you a senior? An AAA member? A veteran? Ask the hotel if they can give you a discount. Also ask about weekly rates.
Haggle:
If all else fails, negotiate. Most independently-owned hotels will bargain with you over the phone, especially at the last minute. Remember that if they have vacancy, they want to fill it, so don't be embarrassed to start low.