Assume that one has begun a PhD program at a university which allows a student to choose an advisor after his/her first year in the program and the student has narrowed down to a couple of potential advisors and has taken at least one course with each of them. 

* How early should one contact potential advisors? Just at the beginning of the program (early bird?) or after 6 months (after taking courses with each one) or right when you need to choose (don't make decisions till you have to?)?