One sneaky trick I've used is to do an internet search (or ask at the hotel you call) for what conferences and conventions are in the area the time you are staying. They usually reserve a block of rooms.
Then, I call the hotels that are conference locations and say I'm with the conference. Then they use one of the blocked rooms, and I get a reservation. The rates may not be great though-- especially if there is a big convention in town.
I did this once for Miami when the boat show was in town. Worked like a charm.
Another tip is to try bed and breakfast's instead of hotels--- lots of people don't think of them.
First of all, call the hotels you're interested in staying at and try to book a room directly through them. Some hotels make blocks of their rooms available to online booking sites. When those rooms are all reserved, it can make the hotels appear to be booked up. If that doesn't work try looking into bed and breakfasts or vacation rentals. Sometimes these places are nicer and cheaper than hotel rooms. They're also less well known, so they may not book up as fast. Finally look at a map. If there are any good size towns right near Tallahasse find out if their hotels have vacancies.
Hang out in a singles bar and pass out candy to strangers. You should be able to spend a week or so with new friends everynight.
you probably have done these, but i'll list some thoughts. have you tried hotels.com, expedia.com, travelocity.com? sometimes they will have hotels when the hotel itself will tell you they are full.