If you are going to powder coat it the Powdercoater will sand blast it as part of the prep he does as his service, so you don't have to do anything. If you have any, and I mean any, modifications planned for your frame do them now. That means if you are going to cut off any tabs or weld on any new anchor points, cut any tubes, anything at all, now is the time to do it and when you've done your metal work just squirt some rattle can paint on it to stop it from rusting. The Powdercoater will blast that right off too.
If you've got any kind of number plates, like the old Honda's had, then you might want to remove it before you send the frame to the Powdercoater, if you've got a swingarm rebuild you'll want to do that before the PC too. Powdercoating frames really is the way to go, its way better than painting, and I'm saying that as a bike painter.