Aside from not getting paint on the discs and pads, do you have any advice or tips when preparing and painting brake calipers?
You want to make sure your calipers are PERFECTLY clean. If they're rusty, clean them off with a wire brush attached to a drill. Use a degreaser to make sure the surface is perfectly clean - otherwise you risk having the paint peel/flake in the future. Then use a proper brake caliper paint that is rated at extremely high temperatures. Do several coats and let it dry between coats.
In addition to what NoCarrier said, realize that brake fluid is extremely hard on paint, so be careful about doing brake fluid flushing. As far as getting paint on the rotors, I've heard that some people prepping cars for shows will paint a stripe on the caliper, so it may not be as big a deal as you think. :-)