Here's a recent related question:

http://www.michaelandpaula.com/mp/AzoForum...&GID=1908&CID=1

I've only tried Michael A. Smith's formula, so I can't comment on the others, but I've heard they don't last nearly as long and don't work as well. Check out his site for the formula, developing times, technique, etc.

http://www.michaelandpaula.com/mp/Azo_Formulas.html

My $0.02.