I got bumped on a flight recently. I should have pushed more, they probably would have upgraded me.
Delphi has written 2 entries about this goal
My strategy for this is simple- never refuse the offer to be bumped. It hasn’t happened to me yet, but several people I know got free first class seats due to having to take a later flight. I don’t think I’ll be able to justify the expense anytime soon, so this is my best option.