Or simply do a search for Stoner's Guide to Memoury Optimization.
If you do not wish to go that indepth, try MaxMem. You can download it at BlackBerryTrade.com
Please note that if you wish to backup the files you are deleting, you need to do so independently. This program does NOT offer this feature. It will, however, allow you to delete unused themes, PTT, VAD, preinstalled media, etc.