A Natural Scientist has a great series about joining a lab which is a must read for those thinking of joining a lab; it is far more detailed than my one post on the matter, more eloquent and thus more useful. (part I, part II). It also includes a first hand account of what happens when you choose a lab that is not a good fit which mirrors what some of my friends in graduate school experienced. I have also heard of horrifying post-doc experiences including a post-doc that switched from one such lab into the one I am in. The stories that post-doc tells are still shocking to hear.
Updated: part III.