I put mine in my pocket only when I know I will have a lot of usage and no power supply
Posted from my CrackBerry at wapforums.crackberry.com
Not a good idea to keep lithium ion batteries in your pockets. There are possible hazards to the heating of these batteries leading to the leaking of the acids inside. Try the backback idea though. Anywhere in close and tight contact with your body is probably not too safe of an idea.
I bought a mini charger and I charge my spare and then switch it out when my battery is 1/3 to 1/2 full. I do this so that I don't have to worry about having to put my phone on the charger and my batter always gets a full charge.