Auto parts store sell a special wire brush. Basically, a short
pipe with wire brushes on the inside that clean the terminal.
You can also get some little felt pads that go under the post
to reduce corrosion.

Example of a cleaning brush:

http://www.vehicle-wiring-products.co.uk/VWPweb2000/photos/1450.jpg

(that one looks a bit fancy, I've seen simplier ones that are
probably cheaper.)

I have also seen many posts here about battery cable corrosion so
check both ends of the cables in case they are corroding at the
other end, too (disconnect at the battery to do this).

Don't forget to wear safety glasses when working around the battery :)