I have been on the 200MB plan now for almost a year, normally use about 50MB because I work from home and generally I am on Wifi 95% of the time.
I have 2 Apps that help me keep track of usage and keep me from using data when I really don't want to, like when riding my motorcycle, etc.
Toggle net switch widget - uses to turn off/on 3G. Wifi, BT, etc.
3G Watchdog - tracks usage, keeps daily. weekly, monthly totals, alerts if you get close to your limit.