Hi there Gavin.
FIRST thing to do (which you might already be doing) is to use Firebug (in firefox, or i think it's in chrome also). Helps you zero right into the css/html code you need.
SECOND thing to do is, when you are testing css, to make sure that gzipall.bat is being run each time (if you have combined / compressed ticked in the server settings (webserver procedure, template, advanced tab).
THIRD thing is to make sure you are running from a custom css file, so that you are not editing the actual Nettalk css files.
You might be doing all these things. In that case, maybe a bit more info would be good to try and help you out. Like how are you doing the changing of the css? And when after you make the changes and it "doesn't work" .. are you checking the css in the page to see if it has changed?
Hope that helps.