A good place to start if you get alot of messages and delete them on your berry.... Goto your messages, then open the menu, options -> email reconciliation. Click menu again and choose purge deleted items. This will clear out the deleted items for your email accounts on your BB, sometimes there's a lot of space taken up here.
Also I would hook the BB to desktop manager and go into the backup and restore advanced area to take a look at the individual database sizes. Just make sure you have a backup BEFORE deleting any of the databases so you can restore if need be.
Check in there and you will get an idea of what databases on your phone are eating up space. Otherwise, if you have an ass load of applications, time to remove some you don't use!
Also, as NJ said above, clearing logs, memory cleaner, etc do little to help. A battery pull does a log clear as it boots up, and memory will most likely be re-arranged, so those things do nothing a battery pull wouldn't for stability / memory usage.