As we gradually move toward a cloud-based mobile experience with services such as iCloud, Amazon Cloud, and Google Music, going over your data limits will be easy. Naturally, service providers like AT&T and Verizon don't mind--they'll gladly hit you with a $10 to $30 overage charge whenever you exceed your allotted bytes.
Connecting to Wi-Fi as often as possible is key to minimizing cellular data usage, but there are a few other best practices to follow. Once you have those on lock, monitoring your usage in real time will help you avoid any overage fees. Here's how:
Take control of data usage
First, cut back on data-hogging apps
Minimize use of streaming services. Apps like Pandora, Netflix, and Spotify will quickly eat up your data allowance. Even a low-bandwidth app like Pandora will kill as much as 30MB per hour--that's a lot of data, especially when you're on an introductory 200MB plan. Save most streaming for when you're on Wi-Fi, or check out a service like Slacker, which lets you cache music stations.
Download apps on Wi-Fi. While your friends may have convinced you to get the latest with Friends game, hold off on downloading it until you've connected to Wi-Fi. Some storage-hungry apps (like Dictionary.com) will immediately ask you to connect to Wi-Fi upon downloading, but you should follow this practice with all apps.
Turn off needy apps. Push e-mail, app notifications (especially Facebook--you probably get those frequently), and GPS services should be disabled whenever possible. These needy services constantly communicate with the network, so you'll save a lot of data by manually enabling them.
Now, monitor your data usage
Android: Check out MyDataManager (free). The app tracks your data usage in real time and offers you a detailed breakdown of how much data each app is hogging. This way, you can identify which apps use the most data. The best feature? Data threshold alerts. MyDataManager lets you set thresholds (like 200MB, 1GB, 2GB, and so on) and get notified whenever you pass a threshold.
iPhone: Users recommend DataMan (free). The app tracks your data usage in real time, and while it doesn't offer details about which apps are using the data, it does let you set thresholds and receive alerts when you're close to hitting your data allowance.
Also check our more detailed advice on how to track Android data usage and how to prevent iCloud from eating up your data plan.