I assume your not rooted. Either way there are some apps. In the market that will clean your system cache, some free some paid.

Clearing the Cache on your applications might help with a general system slow down.

It can be done without an app by going into your setting < Applications < Manage applications.

You have to select each app one by one and clear their cache, which can take some time. You can also manage your running services in this menu, tuning off things your not using will help free up some system space.

clean cache works well if your not rooted

Cachemate is that way to go if your rooted.