trimming masking problem

i use tamiya maskng tape for masking and i have problem of masking round surfaces or small surfaces. howdo you mask rims or things round like that? lots of explanation was made about using x-11 knife to cut but i cut the paint under too. also i cant draw perfect round lines with kinifes so it still looks poor.

can someone show steps of a round masking process?(pics)

http://www.arc6.prohosting.com/Tools%20'n'%20Tips/0001-1000/001-100/051-060/TNT055_Circle_Cutter_Bamford/tnt055.htm

Wow! :) Good question, good answer... I was just starting to wonder where I could find something vaugely like that. I was already thinking I'd have to trace a circle (or print one out) and cut it out by hand... That thing's cool! :D

(And I fixed the link.)