Check out PNF stretching, or aka isometric stretching. Pavel Psatsouline or Thomas Kurz are good, the former being expensive though. Google searching will probably get you the information you need.
Basically sit in your stretch i.e. seated, one-legged hamstring stretch. Stretch as comfortably as possible then gradually tense the leg for about 10-30 seconds. Release and relax and at the same time strech forward while breathing out (Although you are breathing normally throughout the exercise!). Repeat until you get bored. Do this about 2-3 times a week. You can do normal stretches on your off days if you like, but its not necessary. If you do you might need to spend 10 mins, or more in the stretch. You are waiting for the muscle to relax from the tension. The thing to look out for in this stretch is to keep your back straight not the hunch forward, as many people do. You want to be stretching your hamstring not your lower back! Therefore look forward at the foot you are stretching, and try to touch your nose to your toes.
I have found it far more effective than static stretching. Nearly in both splits after 2 months.
Its your nervous system that stops your range of motion - not tight muscles, as previously thought. For example a patient knocked out on an operating table has full range of motion whereas conscious they do not. Another factor is that if your muscles are weak then your nervous system kicks in and inhibits the muscles from extending to their full range of motion. So by doing the pnf stretches you are strengthening the muscles. Very clever!
Coffee does not help, so avoid before training, and keep well hydrated. Also according to Kurz cycling tightens your hips up, so it is counter productive if you are wanting to achieve the side split.
Good Luck! You'll be like Jean Claude Van Damme in no time.