Download Opera Mini (not Opera Mobile mind, but Opera Mini). Use this instead of the default browser when on mobile and use the standard one when on WIFI.
Turn off auto update for all your apps in the market. Download AppBrain and use that to manually check for updates, or let AppBrain regularly check for updates but set it to only download them when on WIFI.
Make sure programmes like Google Listen are set to only download their files when on WIFI.
Syncing does not cause any big drain unless you have an unusually large amount of feeds or email accounts or something. Start using push email if you can though, use the Gmail app for this and disable the auto checks for any pop accounts you have in the standard email app unless you really need them. Checking every 5 mins for your emails can be a drain I suppose. I have all other synch checks running at default except the stock checker which I have on manual as I don't use it.
Do all this, and you'll see your data usage plummet. You won't have to drop much in the way of functionallity to do it, either. Opera Mini in itself will be a big saving in your data allowance munching, the other tips are just common sense.
