[Kevin Sevcik]

I think the easiest way would be to use the plastic window motor coupling provided by FIRST. It's a ~4in diameter black plastic disk thing with a coupling on one side for the output gear of the motor and a keyed hole on the other. So you could attach a keyed shaft to the disk or something. Alternatively, you can bolt things to the plastic disk. All in all, highly convenient. If you're missing them, they can be ordered from MMH Ventures.

If you want to be really fancy and you have the equipment, of course, you could just mill out the center of the disk to slide straight onto the gear with some fancy CNC work. I'm guessing you don't have access to this, though, so go with the FIRST provided coupling.