There are great explanations with pictures in the books
Essentials of Yarn Design by Mabel Ross
and
Spinning Designer Yarn by Diane Varney
I know it takes mohair or ingeo or a few other fibers to get the loops to loop correctly. I expect it may have something to do with the crimps per inch of the fiber -- if I had to guess, I'd say look for a fiber with not more than 3 crimps per inch. The drawing shows you what I mean about crimp length, as opposed to fiber, or staple length... (picture © 2006 The Bellwether, from Spindling: The Basics, used with permission.)Here's a discussion page I found with a similarly brief description of boucle, but with a ton of additional feedback and further explanation that really helps explain things:
http://www.knittersreview.com/forum/topic.asp?TOPIC_ID=49689&SearchTerms=,boucle
They point out there that the Winter 2005 Spin-Off has an article on novelty yarns, including boucles, by Judith Mackenzie-McCuin.
And if you are anywhere near Puyallup WA, on April 20th, 2007, then I can heartily recommend that you sign up for the class "Boucle Without Tears" by Diane Bentley-Baker at the Shepherd's Extravaganza. She's a terrific teacher and funny to boot!
(this posting inspired by recent questions on spindlers and SpinningOnTheEdge)