Sourdoughs starters methods vary.  Here is one [1] from a Julia Child program featuring Joe Ortiz

Always good to know if you can't get a starter started, try a different method  (but please don't think you are capturing yeast from the "air," they come from the flour)

http://youtu.be/gEP3QW-V0sw [1]

I haven't tried this myself but if you do, come back and comment,  Please!