After just doing some touchup work on something a while back, I think the best tip is to get some power tools involved. Even a cheap random orbit sander will make a huge difference in time compared to the hand sanding block. Once you get through the coat(s) of paint on the big areas and start working on anything smaller then it makes sense to use the more careful touch of hand sanding. Someone who has done a lot more of this than I may have a good recomendation for the right tools at each stage, but I think just starting out thinking with some power tools involved will save a lot of arm work. Unless you are looking to get a good arm workout that is, you can build up some good tone working a sanding block.

(Wax on, wax off!)
The trick with power tools is to learn their pace quickly. You can move through paint fast with them, but also move quickly through metal that doesn't need to be taken off. Knowing when to stop is the key.