Any ideas on taping off a good circle?

3M makes a vinyl tape (blue) is very plyable and easy to run without wrinkles. Similar feel to electrical tape. Can find it at automotive stores or Urban Paint in Tonawanda.

I used that for my leading edge:
http://photos.e46fanatics.com/data/3936/wheel_polished_03.JPG

then just used masking tape to do cover the rest:
http://photos.e46fanatics.com/data/3936/wheel_polished_05.JPG