Tree Rat has a real nice way of doing it.
The way I used to do it (Now I use Tree Rat's way) is that I would put a light pencil line where I wanted the crown to start, then I would wrap blue painters tape around the shaft at that point. Get a scrap piece of paper to wrap around the tape to protect the shaft from overspray. Once done painting, remove the tape. If you want rally good results, DO NOT buy the paint that dries to the touch in 20 minutes or less. The paint does not have time to flow together. Get the paint that dries to the touch in 2 hours. You will be much happier with the results.
When you are done, wait 24 hrs before doing anything else to the arrow. DO NOT rush it.