It just dawned on me that the original question was "what's the best way to shim a kneck?'.
We've covered the full range of what to shim it with but not how to shim it.
I'll try to explain how I shim a neck and comments are welcome.
1-Determine which way neck needs to be shimmed.
If the bridge is too low to make the action lower with the adjustment screws in the saddles, shim the neck pocket at the very end of the neck. The closed end of the neck pocket.
If the bridge is too high , more than about 1/8" of the screw out through the bottom of the saddle, shim the pocket at the open end of the pocket.
2-Crank down the tension on all the strings. Remove the screws holding the neck to the body. No need to remove the strings but a capo on the first fret will keep the strings under control.
Use shims of your material of choice. As Hambone said, make the shims about 1/2" to 5/8" wide. Make them as long as the neck pocket is wide.
Be prepared to take the neck off two or three times
as you tweek it in by using the right number of shims. Don't use a drill for a screw driver. Its too rough on the threads in the body. Hand tighten ONLY. When you are starting the screws into the body, use a light touch for the first three or turns. The idea is to make the screw go back into the same threads that it came from. If the screw has to cut new threads each time the wood will become "screw sick".
3- Lay the shim(s) in the neck pocket either right at the back of the neck pocket or right at the edge of the open end of the pocket. Always make sure that the screw doesn't go through the shim and that the shim is outside the screw pattern. I always start with three shims but I expect to take the neck back off at least once to fine tune the adjustment.
That's really all that there is to it. Just work carefully and have the patience to do it right.
I'd like to mention something that's just a bit off the topic but I believe that it needs to be said.
When advice is offered by me or anyone else, it's to be understood that you are somewhat mechanically inclined.
If you don't know what an allen wrench is, you probably will be better off to pay someone else to do it and let you watch. Bob Villa can work one 27 different ways.
Sorry for being so long winded.
Pkr2