I learned how to make the optical illusion by studying this collage professors lessons. They work good on the Internet but not so hot in actual print. I've made lots of different ones and tried lots of different colors but none of them can make the transition to print well.
Here is a link to a website of Akiyoshi's. He is the master at these and has a page with a little lesson on why they work and how to make your own. Look under the first group of snakes for a PDF that explains how they work.
Or here (PDF Warning).
http://www.ritsumei.ac.jp/~akitaoka/index-e.html