There is a part of Tae Kwon Do that many students tend to over look but can be the biggest factor in determing how well the student manages in perfecting many of the kicking techniques. Especially the more advanced kicking combinations and jumping kicks: Flexibility
For some people flexibility is never an issue as they are naturally flexible while others may struggle with it. Improved flexibility doesn't just mean you can kick higher, but kick higher and deliver more power as relaxed muscles won't be working against themself to get the leg up to the height you are trying to kick at. It also improves balance which helps when moving from one technique to another and better balance means you can kick/punch quicker and react quicker. Very useful in delivering strong kicks and punches but what I think is even a more important benefit is making it easy to block and evade an opponents techniques.
There are different types of flexibility. Dynamic Flexibility which we practice when doing leg raising exercises in warm ups and of course while doing most kicking techniques unless you hold the leg in position which then requires good Static Flexibility. We practice static flexibility when sitting down doing leg stretching.
To learn more about flexibility training and valuable routines that will help all levels check out the following website. There are plenty of free articles and links and the video/dvd while not produced to Hollywood standard contains great material well worth checking out and just may change the whole way you think about flexibility.