I have found there to be somewhat of a zen art in getting bumped.. which includes:
1). When getting to the gate, see what sort of aircraft is at your gate, then eyeball the gate area and see how many people are there (as you get nearer to boarding time). Sometimes the crowd gives away the fact they are overbooked, depending on the size of the aircraft. I thikn many of us have walked down one of ATL's terminals and seen a huge throng of people jammed into one gate for a 757 or 767 to the west coast, etc.
2). Don't wait for the gate agent to announce for volunteers. Use #1 and other techniques to predetermine if they may need some, and then, when they have a slow point in the line, inquire about volunteers preemtively.