How big is your current hard drive? Do you have another hard drive? Do you have funds for another hard drive?
There's not much you can delete to free up disk space in OS X short of deleting applications you have installed and files you have downloaded. You can try cleaning out the system caches with a utility like Cocktail, available from versiontracker.com, however i don't think that would amount to much more than a few megabytes or even one hundred megabytes.
OS X likes to have free disk space to work with. It's good in general practice not to let your hard drive approach its capacity, since hard drives that are full do not perform like hard drives that have some free space on them. How much space available do you have? Are we talking hundreds of megabytes or less?
If you have the funds, I would spring for a bigger hard drive. Keep documents, MP3s, movies, etc. on a separate hard drive and try to keep some free space available on the system drive.