One of the biggest things that chews up memory for me is temp memory (i.e. cache) used by apps. Cleaning up app cache has been the best way I've found to reclaim space.
I use a free app called App Cache Cleaner... install it, run it, then hit the sort by button so it sorts the apps by the amount of cache they are using. You can then select the apps that use the most space and go into the android application setting dialog to clear the cache for each app.
Also, un-install any apps you aren't using.
If you are still low on space you can root your phone and install a custom rom most of which are optimized to reduce the amount of memory used.