This is so basic that almost every intermediate level SEO will already get bored with. Maybe even the beginner SEO, or maybe even the non-SEO web designer or web developer knows this already. But I just posted in my SEO tips how to do robots.txt. But aside from showing you how to do a robots.txt, my robots.txt shows you how you should not do a robots.txt. Note, this changes also at random per page reload, I hope I arouse the SEO people out there.