LOL. Install 3G Watchdog, you can set it to alert you when you are close to your data usage limit. It will also show you your daily, weekly, and monthly data usage totals.
If you like to see how data you're using on *wifi*, you can install Data Counter Widget, which shows both mobile data and wifi, whereas 3G Watchdog only shows mobile data. But DCW doesn't have any alerts.
Set your phone's wifi to Never Sleep, so that if you're home and connected to Wifi, it stays connected to wifi. If you don't do this, when the phone sleeps, it will connect to mobile data instead.
One caveat...because there's sometimes a delay between when you use data and when the data is captured by AT&T system for billing, there may be issues.
For example, my 3G Watchdog says I've only used 40 mb of data (which is correct as I've been on mostly wifi since the beginning of my billing cycle, 10/24 - and I haven't streamed any music since then), my AT&T *DATA# says that I've used 225mb!!
So just be aware there may be issues with end-of-cycle data rollover because of reporting delays.