I just purchased Elastic Steel and I have to say, it shows a great deal of potential. I have been a MA for 11 years now and I have learned a few things along the way.
First of all, everyone mentioned warming up before being able to achieve higher kicks. I think it is important to explain that warmping up isn't just stretching. It is very important to warm up your body with exercises like running, jumping jacks - anything that can be done in a short amount of time that will warm up your muscles before you begin your stretching warm up. The fastest way to an injured body is a cold one.
That being said, it is also important to warm up all your muscles. Core muscles especially. People always work on their stomach muscles but many forget about their back muscles which are equally as important.
I have the Tom Kurtz book. I found it to be just ok. Nothing new. I also have just about every Sang H. Kim tape out there and while I love him and his workouts, his stretching tape also left me needing more.
I have also puchased the series that uses Kettlebells, I am sorry, the name escapes me for the moment. His phylosophy which I found interesting as well as effective, is to stretch, tighten the muscle you are using, holding it for about 10 seconds, then let it go and stretch a bit further. This can easily be done with whatever stretching you are presently doing and the results do come.
Back to the elasticsteel. Every stretch is explained which I really like. Knowledge is power. He also stresses strength which I think is key to high kicks as well as a good split. And finally, each exercise works both sides of the leg. I often find most of the stretching tapes I have, only work the inside of your thigh, leaving me in pain with little to show for it.
Someone mentioned partner stretching, having your partner go down into a squat as you place your leg on their shoulder. This is also a great stretch for higher kicks. However, don't forget to work all the angles of your leg. Let them stretch you first, facing them. Then again with your leg turned to a round house kick, again, with a side kick and finally, facing away from them, in a back kick stance. That way you work all the muscles from your hip flexiors to your bottom.
In the past 10.5 years I have never been able to get that perfect split. I have always been just a little off - but in the last half year, following the strengthing method as well as warming up my muscles before I stretch and paying close attention to stretching the inside, outside, back and sides of my legs, has finally brought me to a full split - and more relaxed, powerful and higher kicks.