BB is correct about holding the button while changing batteries (I was going to mention that the battery also holds the button in place). However, if your phone is like mine, when you accidentally drop it, the battery comes off. If so, then your button and spring may likely go flying off.

As far as the rubberband thing, I cut a standard office rubberband (not one of thos big thick things) into two pieces about 1/4 inch long each. I laid them on either side of the hole the main post for the battery button goes into. Then I carefully reinserted the battery button so that the rubberbands were on each side of the main post between the post and the side posts. If you press down you get kind of a spring action. You are only looking for enough to make the button raise back up and engage the battery. I'm not sure how else to describe it. There is another thread on this issue somewhere in this forum where I got the idea so it might explain it better.