Be at the gate at least an hour before scheduled departure time. Get on the list as soon as possible.
I have turned "no" into "yes" by offering alternative flights that had not popped up on the agent's screen. Know the alternatives.
I was once at LAS headed home to DCA and all flights east were also oversold. I asked the agent to send me to LAX and then on a redeye from LAX to IAD. They did it.
If you can be flexible in any way let the agent know. Make it easy for them to bump you (alternate airport, for instance).