http://www.automotiveforums.com/vbulletin/t56889.html
It is most important that you use a sharp blade to cut the masking with. The blades are cheap, and I have one knife handle with a brand new blade just for cutting masking tape and decals. When the blade is too dull for that, it's still sharp enough for other work, so I transfer it to another knife handle. If you can find them, buy a box of 100 blades, it is much cheaper that way.
The right tape is important too, Tamiya's tape is excellent.