What's your app count? Do you have any with really large memory footprints?
One thing to try is clearing some cache, like internet browsing cache:
1. First go to Settings -> SD card & phone storage and note the "Available space" at the bottom
2. Then go to Settings -> Applications -> Manage Applications -> Internet -> Clear cache button
Note: you can do this with other applications too (search for ones with very large values (in MB) listed in the page where all of the applications are listed); I don't advise to any "Clear data" unless you have a good reason to (anyone know of any?).
3. Check your memory again like in step #1
4. Is the phone any more responsive?
By the way, there are several cache cleaners out in the Market (several are paid apps)--you'd be best advised to search these forums for other folk's recommendations regarding what works best.
Hope this helps.