It is important to separate stretching from limbering. If you are looking to actually increase your basic stretch capability of your muscles, then long, slow, steady stretching is best, this also leverages muscle memory if done correctly.
Dynamic exercises are typically best for limbering, where you wish to warm up your muscles and have them 'recall' your stretch limits. I think using dynamic stretching, especially bounce type stretching, to try and increase your stretch capability introduces the risk of torn muscles, ligaments, etc. So it may be quicker, but can have a higher long term cost.
The splits is one of the hardest things to improve on once you are an adult, but it can be done. It is best though to go for all around flexibility, this will also reduce your risk of injury and help increase your perceived fitness (since many physical things will be easier).
Try working on your calves with the classic runners stretch - face and lean on wall, one leg in front near wall and bent at knee, one leg behind (2-3 feet) with heal on floor and straight, now push hips down to stretch rear leg calf. Remember to breath and stay in position for 1-2 minutes to relax into stretch.
This stretch will relieve your hamstrings somewhat, which you can work on next.
Hamstrings are best done seated and one leg at a time. Keep one leg tucked in (foot to thigh) and the other straight in front, toes up. Lean body over straight leg and reach hands towards foot. You can use a belt to loop over arch of foot and pull yourself down (don't pull on tops of toes though, as tenses calf). Now make sure your back is straight by looking up and trying to push your chest forwards, this will increase stretch on hamstring and prevent damage to nerves in your lower spine (common fault on this stretch). Remember to breath and hold for 1-2 minutes.
These should both help increase front kick flexibility.
A few things to remember for any stretching:
1. Never over do it, the possible damage can be far worse than being inflexible (BTW been there done that!)
2. Remember to breath deep, this feeds oxygen to the muscles and also you can usually stretch further on a breath out.
3. Hold for a good 1-2 minutes, this helps let the muscles actually relax (tense muscles do not stretch well!) and helps enduce muscle memory.
4. Try and avoid bouncing as it can tear already stressed (i.e. stretched) muscles.
5. Make sure you are warmed up if trying intense stretching. Either do some light dynamic exercises for 5-10 minutes, or take a hot bath.
6. Try not to heavily stretch the same muscles two days in a row, muscles need time to recover from any exertion (as in weight training). Apparently Vitamin C can help muscle recovey(??)
One last thought is to look into Yoga which includes many very good stretching exercises.
Hope this helps and good luck.