How can I clean up my expansion card?

A. Uninstalling applications properly will get rid of most of the files that use up your 'space', so you're probably not wasting very much. There will always be exceptions though, with small configuration and settings files left behind on both internal and external disks and I'd recommend using a File manager (built into every Series 80 device) - FExplorer on Series 60 - to get to know the filing system on your smartphone. Think of it as getting to know what's under your car's bonnet.

In particular, look out for folders within \System\Apps with names of applications that you know you've already uninstalled. You might also find some relevant files in the \Nokia tree (on S60) or \My files (on S80), although again I doubt that you'll save many kilobytes - it's more a peace of mind thing!