A really easy trick I learned from someone that works very well, especially when the wheel is molded in white, is to take a good razor blade and carefully scroll around the rim where the angle changes below where the rim meets the tire. If you carefully angle the blade you should be able to scrape the paint off with little effort and it leaves a near perfect white ring around the wheel. Then use a red or yellow or whatever colored Sharpie to color the line. The Sharpie won't smear or runoff when you clear it. It puts the ring inside the wheel a bit which is a more accurate look I think.