Delete/uninstall all of your apps (or those you installed just prior to the leak), then one by one reinstall them fresh (reDL them, do not backup/restore them). Use each app for a day or so until you notice which one is draining the memory.
Posted from my CrackBerry at wapforums.crackberry.com
Pretty much what Isuben said. Have to go through each app one by one. Also, what OS are you running? .151 has the best memory management to date so if you arent running that I would highly suggest making the switch. Also, what apps do you have installed on your device and how many are running in the background? Themes?