Today's Android smartphones have pretty reliable data monitoring tools built-in, but since this is Android we're talking about, you're well aware that there are many more ways to approach handling your monthly traffic. We'll introduce you to one of them - Data Status by Flavour Monkey Apps. We like it because it shows timely data usage information inside a convenient meter in your device's status bar.
Install the app, start it, and tap the "monthly allowance" button where you'll punch in your data cap information. Then, write the start date of your monthly billing cycle. This is how Data Status learns your data plan and when it should automatically reset your data counter. You can also enter the amount of data you've already used up so the counter starts off right.
Following the simple setup, your remaining data for the billing period will be shown from a small meter to the left of your status bar. For additional information, you can pull down on the notifications drawer and find out how much data you've used up, and how much remains. Gotta love having easy access to information without it getting in the way!
Data Status is free on Android.