You wouldn't believe how many times I've discovered my volume was turned all the down because of that. I guess I just need to learn where to put my fingers when I pick it up.
I had the same problem with accidentally turning down the ring volume, missing calls.
Then I discovered "ring toggle". It is an app available in the "market". Search for "ring toggle", install it. With it activated, if you change the "ring volume", a window pops up and asks if you want to accept the new setting. A count-down timer [user adjustable, mine is set for 5 seconds] returns the volume to its former preset if you don't "accept" the new one.
Problem solved.
DON