With a sample size of ten permanent markers, and some spare time today at work, I have determined that the worse a marker smells, the more permanent its markings are. So when you are out shopping for a marker, be sure to buy the one that smells the nastiest.

This is probably due to the fact that stronger solvents smell worse than weaker ones (because they act more on the sensory tissue in your nose.) Therefore, these solvents dissolve the surface slightly and bring the ink into more intimate contact with it.