You can take as many consecutive bumps as the airline's overbooking and your charm permit. Once you have confirmed that the flight is, in fact, overbooked, ask to be put on the volunteer list. Politely let the gate agent know of your interest. But don't pester him/her. Don't check any bags. Wait until the last minute to board in case they do need your seat. And stay within reasonable proximity of the gate agent so he/she can find you without too much effort. Good luck.