Panaracer tire liners are made of Kevlar and are remarkable for their ability to prevent punctures. I have tried to push sewing needles through one of mine and could not do so without placing the needle in a vice and by pushing very hard; I can't imagine one of the dreaded Goatheads ever getting through.
We never hear much about these wonders; perhaps because they are expensive ($18 or so each); perhaps because no one pushes them; perhaps because many say that they cannot be re-used, which is not true.
The Panaracer liners have an adhesive back to hold them in position in the tire. They can be re-used by simply spraying a coat of contact cement over the previous adhesive. --- They work, last the life of many tires, don't cause flats, and, I at least, cannot tell whether they are even in the tire (they are very light and compliant).
There are many sources; do a google search. Here is one link:
http://www.amazon.com/panaracer-Pana.../dp/B003UWIGRY
Joe