Different kinds of stretches. When you pull your toes back, you stretch the calf and the hamstring. When you point your toes, you stretch the tendons that go over your knee, and down your leg. For some advice on kicking better vertically, start doing the splits. I had pretty good kicks to begin with, but my Sifu told me that if I do splits, my kicking will improve by tenfold. Which it has. It took me about 4 months to ALMOST touch the ground. Right now, my crotch is about 5mm off the ground. My kicks are good and high, fast (working on making them faster), and accurate. When I throw a roundhouse, people have told me it looks like I'm doing a split in midair kind of.
So yeah, start doing the splits 5 times every night for about 4 months, and your kicking WILL get better. Need any advice, just message me. I'm really into Kung Fu, and I'm into fitness and flexibility, and obviously VERY flexible.
2 years in the art of Tien Shan Pai Kung Fu (-blue sash-)