How big of a concern are "hot spots" or clumping when you mix individual powders together? Does this happen a lot? I'd like to try capping or making a batch of powder in the future and I want to make sure I get an even distribution of the ingredients.

A mortar and pestle will get rid of the clumps and mix pretty good. If its a really really big batch (which I don't generally recomend unless you've tried the mix in a smaller quanitity first) then just get a big tupperware container, put the lid on and shake the crap out of it.

I used to grind my charcoal for pyrotechnics by sticking it in a cofee can with some marbles in there (to act like pestles) duct tape the lid on, tie a towl around it for padding and stick the works in the dryer on the no heat setting.
Works great but would probably be way over kill for what you're doing.

The mortar and pestle sounds like the best option, although that coffee can idea is pretty cool too. Thanks.

I thought about using the spin cycle on the washing machine for a centrifuge once but figured I'd end up ruining the laundry

That would be a lil overkill.