My iphone 3GS phone's volume/silent mode keeps changing automatically, help!

My iphone 3GS is about a year old now, and for a couple of months i have been experiencing my silent/volume mode that quickly changes, even without me touching it. A friend of mine also mentioned that he was experiencing the same problem and took it in to apple and they simply replaced it, saying it rarely happens...

I am a college student and rather not have my phone ringing in the middle of class. Is there an easy fix for this or do i have to go to an apple store?