There have been some pretty good posts here explaining this process, and here's a link to a pretty good write up that explains how to do it.
http://www.motorcyclistonline.com/howto/rsuspshim/
The reason to shim a bike is to alter the geometry of it. By putting more weight up front, you'll get some better handling. However, you don't want to go too drastic, 6mm should be the max, otherwise you're going to experience some difficulty in handling.
I haven't yet done this myself, but I plan on doing it as soon as I get some time. I'm not going to be ordering any parts for this, I'm just going to go to Home Depot and get some washers that will fit my bolt. I'm also probably going to be testing this in increments, maybe 2mm at a time until I get it to a point I like, and am comfortable with.
Jay