http://gymnastics.about.com/od/trainingadvice/ss/frontsplit.htm
http://gymnastics.about.com/od/trainingadvice/ss/centersplit.htm
I found these have helped. You can also get DVDs such as Aletheas, and Felix Cane's, both of which I have and find useful for both back flexibility and the splits.
This is one of the reasons you see so much discussion on here about attaining the splits, and about flexibility. It's a difficult thing and has to be maintained, and some people find themselves working against certain difficulties because of the way their body is made.
My one piece of advice from personal experience: Do NOT stretch as far as you can, regardless of pain, in order to achieve flexibility. This will actually reduce your flexibility and set you back way far! Reach till you feel a bit of discomfort, and stay there- count to ten and focus on relaxing your muscles. You may feel the discomfort release, or fade, and when it does, you can move a little farther into the stretch. 60 seconds (about a minute) is what I find best, but… some people wait for longer, and some don't need to go for as long. Just be careful and learn what works for you!