um... okay, well, a lot of the other (more SANE than Dave's) posts seem to have hit the nail on the head. A team name, logo, number, or even small picture should be sufficient; additions that are funny or crazy sometimes make your buttons higher in demand.
We've ordered our buttons from here:
http://www.everyonelovesbuttons.com ... and have never had problems or complications. They offer a variety of sizes and shapes of buttons, and are pretty quick with the orders (okay, ridiculously quick... last year we hesitated and needed buttons last minute, and they got 250 buttons to me in FOUR DAYS). Highly recommended over here.