NOTE: This only works if you booked directly with the hotel. If you use a third party (Expedia, Hotels.com, Travelocity, etc. you can not do this!).
Let's say you have a reservation. Before you check-in, pretend you are a walk-in (meaning, you just walked in off the street and do not have a reservation).
Ask for a room exactly like the one you booked (so the value will be the same). Believe it or not, the rate might be lower! Why? Because some hotel owners (especially the mom and pop types) don't use the concept of supply and demand. They actually set HIGHER rate earlier and then LOWER the rates at the last minute because they have a TON of rooms to get rid of!
So, if you are given a lower rate the your reservation, demand the lower rate. If the clerk won't give it to you ask for the manager or owner. Usually in the smaller hotels, the owner is on the premises or is a cell phone call away. They should be willing to work with you on the rate.