How to reduce data usage on an iPhone?

Recently made the switch to Apple (iPhone 4) and ever time I look around, I'm receiving a text from Verizon alerting me that I've used 50% of the data or what have you.

A friend suggested that I go into Settings and turn the "Celluar Data" off.

Granted, because I've now gotten the hang of it I'm not fooling around in the Apps Store or constantly using the Internet.

Still, does anyone have suggestions?

Click the box on each of your apps to operate in the background only when Wi-Fi is avai? Lable.
turn off gps use it only when you need it.same with accu weather. Do you really need to know the weather tomorrow everyday? You can see it on the tv news 10 day forecast