PM issues 15 and 16 has the two parts of my last flexibility article:
Basic take-home point in regards to hamstring flexibility is that you must isolate them from the back when stretching--almost invariably people will allow their lower backs to flex when attempting to stretch the hamstrings. Lower backs tend to be extremely flexible and even hypermobile, while hamstrings tend to be exceedingly tight. It's common to hear that hamstrings are unusually resistant to stretching, but I think this is a product of poor stretching and a failure to actually hit them adequately, not any inherent characteristic of the muscles.
PNF stretching will loosen you up as quickly as anything, just go easy--think of it as being akin to training and make sure you recover in between sessions.
Consistency is important too. Someone in your position will need to stretch daily at minimum to see results that will impress you. I would suggest multiple times daily--some light dynamic stretches in the morning, dynamic stretches pre-training, static stretches post-training, and some light static stretches in the evening. Don't overdo it--these stretches should not be avusling the muscles from the bones--you don't need to be in pain for the stretches to be effective.
Some specific hamstring stretching advice in this thread too: