How do you decrease data usage on an iPhone 4s?

[Muñeca]

So apparently my data usage is at 80% and when my brothers iPhone got to like 98% his phone stopped working well so what can I do for my phone?

[the Plumber]

Use a WiFi connection whenever possible.

Turn off all of your apps that "update" or have notifications, like facebook, weather, or breaking news. You can still use these apps... just shut off the periodic update/ notifications.

There are also some games that run background updates/ notifications like words with friends... go into the settings of all these apps and turn off the data.

This should slow down you data usage quite a bit.