Go back and read the blog post on kicking height and figure out how high you can reliably raise your knee for a stable kick. Now that you know where your effective kicking height is, decide whether you’re OK with that or if you want to increase your range. Most likely this will mean working on your hamstring flexibility.
One of my favorite stretches is a simple one that can be done in a chair. Start by lifting one knee up, stabilizing the leg by clasping your hands under the thigh.
Next, extend your leg, keeping the knee in one place. Notice whether this is easy or if you find it difficult to straighten your leg. Can’t straighten the leg? Lower your knee and work until you can consistently extend the leg without dropping the knee.
If you can consistently straighten your leg at one knee height, now raise the knee just a bit and work again to consistently straighten the leg.
Of course, work on this exercise with both legs, even if you have a favorite “kicking” leg. And this can be done almost anywhere (provided you’re not wearing a skirt or dress). Just one or two extensions at a time can make a big difference in the long run.