If you are bumped from a flight when you travel, it’s very important to know your rights.  If you are involuntarily bumped, it’s even more imperative. 
The rights you are entitled to depend on the time that the airline gets you to your final destination, compared to your original arrival time. (Individual airlines may give more, but these are the minimum requirements that you are entitled to by the FAA if you are bumped from a plane involuntarily.)  
You are not entitled to any compensation at all if the airline gets you to your final destination within one hour of your original arrival time.   You are entitled to the amount of the one-way ticket to your destination, up to $400, if the airline gets you to your final destination within 2 hours for national and 4 hours for international flights.  You are entitled to the doubled price of a one-way ticket to your destination, up to $800, if you are delayed longer than 2 hours for domestic and 4 hours for international flights.  (These amounts went into effect in May of 2008.)
If you are voluntarily bumped, and choose to give up your seat, you have no rights at all.  However, there are some things you should do to protect yourself and get the most out of being bumped when you travel.
Get a confirmed seat on another
flight before you agree to be bumped.  Otherwise, you could wait much longer than you expect if the following flights are also full.
When you agree to be bumped from flights, insist on a check instead of a voucher for travel.  These travel vouchers typically come with many restrictions and blackout dates, making them difficult to use.  If compensation is not immediate, get it in writing.
When you volunteer to get bumped, ask that they match the best offer they give someone else.  Otherwise, if you volunteer early, and other people don't volunteer until much later on, they will probably receive much more than you will.
Also, keep in mind any car or hotel reservations that you could potentially lose if you are not able to arrive that day, or any late fees that they may charge you.
If you are bumped, willingly or involuntarily, ask for meal vouchers, hotel vouchers (if you are required to stay over night) and access to the airport lounge. 
Know your rights when you are bumped, so that you are not taken advantage of by an airline for which you are doing a favor by giving up your seat, whether involuntarily or not.  If you are bumped, take advantage of the inconvenience and make the most out of it.

Gina Clark writes on budget travel and financial issues. Visit her blog to learn additional ways to save money on your vacation.