There are many stretches that can help with flexibility. There has been many discussions of different stretches on this forum. Hyper Pro Sami Suddeth has listed some very helpful one that you may want to search for on here. One area you want to pay attention to is making sure your back foot is completely rotated with the toes pointing opposite where you are kicking. This helps open up your hips for a higher kick. Doing side leg raises while holding onto a wall can also help and then try and hold them at the top for as long as you can. Hope this helps...

It also helps if you condition your legs and build up your hip flexor muscle.
By doing leg raises, holding a round or sidekick and doing pulses when locking out your kick, and holding stances all help gain the muscles you need to hold an amazing kick. Make sure you stretch alot when you do these exercises because when you gain muscle, you lose flexibility. And just as simple as practicing holding it. If you set a goal for yourself everytime you practice on how long you want to hold your kick, you will improve quickly.
I know a stretch that helps strengthen your kicks. I will be uploading a video to show this stretch. Bassically, all you do is you go into a door way, and lift up one leg up onto one of the sides of the doorway. As you do this you want to put yourself into a kick position. once your leg is up move your bottom leg as close as you can to that side of the door way so your kick is straight up. To help your balance use both hands to push your body in to make yourself be in the middle splits in the door way. Once you have held this for 30 seconds lift your foot off the wall for 15 seconds; this helps build hip strenght. Repeat this 2 more times, the switch to other leg.
This stretch is hard to explain over writing but I will be uploading this video soon, so you guys can see how this works so many different muscles and helps your kicks.

slow controlled kicks will also help allot. then there is kick measuring it's basically you put a 3-4 pieces of masking tape or anything that will stick, on various heights on the bag ( or even a wall or anything else the key is not to kick it only to get it there )the pieces should be a few inches higher than one another on the bag the lowest piece of tape should be a little lower than ur current kicking height and the highest should be like 6 inches higher or more on the bag with the others in between basically u are going to kick controlled kicks not slow but not full speed so u won't be using so much momentum u'll start at the lowest piece of tape for a few reps on each leg and then u'll skip the next tape and go to the third then u'll go back down then u'll do two kicks with the same leg up and down well use ur imagination :) the reason for that is that static stretching will stretch ur muscle but it won't strengthen ur muscles ( although you do constrict the quads when stretching the hamstrings) and this targets the specific muscle groups and it takes ur muscles through the whole range of motion stretches and strengthens and it also helps with muscle memory, in short it's a great way for building kicking height and strength and power. there r allot of other exercises which are great but as u can see im not really good in explaining it in writing but i tried , and this exercise targets all the muscles that u need for high kicks :) remember warm up before and don't do this full speed for two reasons one because u'll be using momentum not muscle and second because u can really injure yourself. for more stretching exercises special for martial arts check out the book Ultimate Flexibility
by Sang H. Kim he is great!
i hope this helps good luck enjoy

Jacob