I have had good results with Tom Kurtz stretching method outlined in his book:
Stretching ScientificalyThe most important piece I took from this is that static stretches such as holding a split, is not what will allow you to kick fast, and high. This is a different type of flexibilty.
I started to pull muscles a couple of years ago, I never had this issue before, so I started to research it a bit.
After reading this book, I took 2 weeks, and every day, 3 times a day did 20 reps of dynamic stretches, (also called Stretch or swing kicks)
Front up
In to out
Out to In
side up
at the END of my work out, I did static (held) stretches, (deep horse stance, front and side split positions etc)
After two weeks, my flexibilty for kicking had improved, I was less sore and required very little warm up to kick high.
there is a lot more to it, but if you get the book he includes sport specific sample work outs.
He also has a DVD by the same name.
Good Luck!
Gordon