In your contacts list, there should be several items at the top of the list that start with #. Like #BAL, #DATA, etc. Hit any of those and go to "account" and you can see the days left in your billing cycle and a usage summary of data, usage, & text.
Sign up for the lowest amount you think you could get by with and keep an eye on the data limit. If you start to get close, bump up to the next increment. You can do this as often as you want, and then change back whenever you want.
I don't use much data except when travelling, because most of the time I have wifi access. So I got the 1 GB plan, and just keep an eye on it if I've been using more than usual.