Few of us can kick high as well as we'd like. I would like to offer a few tips. This is not an exhaustive list by any stretch (no pun intended), but it may help you out.
- Prioritize your dynamic flexibility. Many people focus on stretching by sitting on the floor or in a near -split and holding the position; this is nice but shouldn't be your main focus. Instead, stretch by swinging your leg and quickly moving it in and out of its farthest range of motion. Do mae keage and yoko keage (leg swings to the front and side) in sets of 10 or so for each leg. If you're going to do static stretching, don't do it pre-workout. Do it at night, with cold muscles. Holding stretches weakens the muscles - stretching them cold will do more to lengthen the tissues.
- Get stronger abduction (spreading your legs apart with force). Try simple exercises like slowly kicking, as high as you can, while wearing ankle weights or with bands hooked onto your feet. Do NOT do full speed kicks with heavy weights - that's asking for trouble in various ways. You need strong hip abductors, and they need to be strong when your legs are spread far apart, a position most of us aren't in very often.
- Get stronger adduction (pulling spread legs together with force). This is good both for moving around and to increase the limits of your flexibility. Basically, weak adductors will contract when they're stretched (it's a protective reflex). To avoid this mechanism you need your adductors to be strong when at the ends of their range of motion. How? Get into a deep horse stance - really, really deep, as close to a split as you can manage - and squeeze the ground. Hold, squeezing as hard as you can, for 30 seconds. Or stand on Valslides or the handles of a suspension trainer, lower into a split, then pull yourself back up with your adductors alone.
- If you have anterior hip pain (pain on the outside of your hips): try stretching your piriformis. Sit up straight. Put your right ankle over your left knee (like half a lotus position). Gently press down on the right knee so you feel a stretch on the outside/ back of your right hip. Why does this help? If that muscle is tight it pulls the femur against the outer edge of the hip socket, limiting your mobility. Loosen it and... voila.
- Kick with a slight anterior pelvic tilt. That means if you throw a side kick or roundhouse kick, tilt the top of your pelvis forward a bit, as if you were sticking your ass out at somebody. This will help align the femur and hip joint so bones don't hit each other as your leg moves high and to the side.
- Practice high kicks often. This one is obvious. Whenever you get a chance, try to have one leg up high in the air. Hold a chair for balance of you need to. You need to convince your nervous system that having a leg way up high, close to the limit of your hip mobility, is not dangerous.
For me, the anterior hip pain is a real issue - it's the limiting factor in my ability to get close to a split. Stretching out the muscles on the back/ outside of the hip, to allow my femur to re-center in the socket, has been a big help in getting my kicks higher.
If you have other good tips, put them in comments, please!