There are scripts to compress CSS on-demand like these.
The most used CSS compressor is YUI Compressor. It supports both JS and CSS.
Talking about JS, Google Closure offers a more powerful set of features compared to YUI Compressor.
Have you tried to use gzip compression. This will bring you much more benefits than the standard compression. Also have in mind that compressing some JS files may bring you troubles. Last year I used internet by broadband connection(GSM network) where the provider`s proxy compressed all the content to lower the bandwidth and this causes me troubles with some sites so test everything you compressed.
Please post and update what optimisation method you choose and what was the benefit.