Erm... I'm assuming you don't want to spend any money, right?
A pretty vague question but I suppose you could start by freeing up space if you have less than about 8GB (if not, ignore me since any more than 8GB free and you really probably won't notice a difference).
Completely reinstalling the OS may give a slight speed increase but I wouldn't say it's worth the hassle.
If you decide you do want to spend some money on it, a larger/faster hard drive could help quite a bit too, but once again, it's probably not worth it.
I guess if you wanted really fast, you could try the hacks that have been posted here to disable Dashboard and Spotlight in Tiger. That would probably make Tiger a lot faster than Panther or Jaguar -- under minimal load, it is faster even with these two running, but you will have better memory and HD space utilization if you get rid of them...