How to not go over my data plan on my iPhone?

I just got the iPhone 4. I only have 2GB. Can you tell me ways to save data and all that? I know to use the internet when I have wifi because that doesn't use up GB?

Just dont get on the internet as much. I only have 3GB and i manage it well.

Mainly just go to wifi. And make sure you actually close every app that uses data (facebook twitter email etc. If you dou ble tap the home button and these appear, then they arent fully closed) as not closing them would continuously uses data. And you can check your data counter by going into settings, then to general, then i think its call like data counter or something like that. Also i wouldnt use the app store or music store unless you are on wifi. And i wouldnt use facebook, twitter etc very much