By far the best shimming/spacer material I've found is 3M Temflex #2155 rubber splicing "tape", available for about three bucks in the electrical section of many home centers and hardware stores. This has no adhesive to get slimy like vinyl tape. It is a roll of very tacky plain rubber that stretches like bubble gum. You peel off the backing and stretch it around the bar/seatstay/whatever until you have wrapped it to the desired thickness. The rubber sticks to itself to form a very solid, yet sticky shim that will not slip and lasts a long time, yet can be cut off without leaving any residue. It also seems to have some shock absorbing qualities which helps with mounts that take a lot of abuse like seatstay-mounted taillights.

There are probably some mounts that it will fuse itself to eventually, but I've used it with TwoFish rubber mounts as well as a few types of hard plastic clamps and I haven't had a problem so far.

It also works well for making brake cable boots and other miscellaneous tasks. Good stuff.