Dish soap works well.
If you want a solid mask, instead of cutting an arch out of a piece of cardboard, I use a For Sale sign from the hardware store. You can "stuff" it a little further in the crease to get that outer most part easier. I do NOT tap it out with a hammer. I laid mine out on the workbench with a thumb tack and string, and made the radius of the arch about 3/4 of an inch longer than the radius of the wheel. This allows you to "lean" the mask in place and get it right to where the tire meets the rim, since holding it flat leaves a little "ugly ring" around the outside of the rim.
When you do miss and get a spot on the wheel (odds are you're gonna do this at least a little), don't mess with it. Make sure it's dry, all the non-stick coatings of choice are washed away, and hit it (gently) with a flat black spraybomb can. After you move the truck you probably won't be able to find that spot again. That works awesome for mud flaps and dropside stoppers as well.