Good suggestions. I like the one from JC the best.
If you have to paint the complete inner part of the wheels , there is another good trick I read about, it includes the use of the airbrush: First paint the rims in the desired color. Then take a coin similar to the size of the inner wheel and cut a circle from a piece of tape using a cutter around the coin. Then divide the circle into 8 equal parts, using a pencil and a ruler. Cut those 8 pieces out properly and use them to mask off the painted rims before painting the outer rims with Alclad Chrome or similar. It's quite some work but I have seen nice results coming from this procedure. It has been used by a guy on a German forum, here is one of his results: http://www.wettringer-modellbauforum.de/forum/index.php?page=Thread&postID=811453#post811453