Change Refresh Intervals
Unless you tell it otherwise, your phone will automatically check for new email, Facebook posts, and other online updates on a regular basis. Even if it detects nothing new, some data will still be exchanged during the authentication process. If, for example, your phone checks for new email every 30 minutes, that's 48 checks per day and the data it uses can add up quickly.
Gmail - If you use the Gmail app, you will have to turn off the Auto-sync option and manually refresh the account to check for new email. This is great for keeping data usage down because it will never check unless you tell it to. In order to change this setting, open Gmail then hit the Menu button and go to the Accounts screen, then hit the Menu button again and go to Account Settings, then uncheck the box for Auto-sync. From then on, whenever you go into Gmail, just hit the Menu button and tap the Refresh button to check for new email.
Email - If you use the Email app on the phone, such as for setting up an Exchange account or something besides Gmail, you can change the refresh interval to a maximum of once per hour, or set it to Never and manually refresh. When creating the account, you'll have this option. Otherwise, open your account, then hit the Menu button and go to Email check frequency and change the interval there.
Facebook - With all its image data and content, the Facebook app can really suck down some kilobytes. Open the app and then hit the Menu button and go to Refresh interval and change the time to 4 hours, or go all out and set it to Never. After that, just open the app and hit the Menu button, then tap the Refresh button to check for updates manually.