Check your MyDocs directory from /home/users. If you put audio and video, PDFs, and such you can rapidly fill the internal flash. Most of these can be moved to the external card.

Look for the amount of memory used by the various apps (Application Manager). It might be one or two fat ones, and there may be ways of reducing the usage.

Look for skins or themes or other things in apps you might not need.

Beyond that, if you hack, look for large files which might not be needed or caches which haven't been emptied (e.g. apt-get clean, check /var/log and other things in /var). Don't delete anything if you don't know what it does.