Don't try to guilt-trip the rep into doing something for you. It will usually backfire.
Don't abuse your favors. One of the Sprint customers dropped by the phone company had been given two free replacement phones even though she hadn't purchased insurance and hadn't paid a bill in six months.
Don't insult the reps personally or the company.
Don't ask for a manager or supervisor as soon as you get on the phone. Reps are limited to a certain number of "elevations" per month.
Don't curse at them or you'll probably be hung up on.
Do have information handy that you think they'll need.
Do be attentive and clearly articulate your issues instead of ranting them out.
Do be friendly.