Two person stretch. Your partner and you face each other. Your partner gets into a horse stance and you place your leg on their shoulder. They slowly come up out of their stance while you maintain a good front kick position. Once you have reached your max on hight, hold that position. They get to practice their stance while you get to stretch, then switch. This is a great win, win situation. If no partner then use your dresser at home, while you do deep knee bends with your support leg, your other leg gets to stretch. Once again a win, win situation. One leg gets strong, while the other stretches, then switch. Good luck.
Those are both great stretches - here are a few more:
Stand on the edge of a stair, with just the balls of your feet on the tread; allow your heels to drop below the level of the stair, as far as you can go. As this gets easier, drop farther; easier still, do one foot at a time (hold onto the stair rail).
Rising kick stretches - this is the dynamic side of the stretches above. From a walking (front) stance, swing the rear leg as high as it will go, keeping the knee straight; this will stretch the hamstring. Do 10 on each side. As this gets easier, kick higher, keeping your foot in front kick position. Vary the stretch by kicking across your body and to the outside diagonally from your starting point.
Sit on the floor with one leg in front of you and one bent in so the sole of the foot on the bent leg is on the thigh of the straight leg, as close to your body as it can get. Keeping the back straight, bend from the hips and reach for your toes; if you can't reach them, grab your ankle or shin, and hold for 5-10 seconds; repeat 10 times on each leg. Ultimately, you want to be able to get your entire hand on your foot, reaching over your toes.
From the same position as the previous stretch, extend your foot into a front kick position, then raise it as far up as you can, using your hands on the floor for balance; hold for 5-10 seconds and repeat 10 times on each leg.
Ultimately, however, the way to improve your kicks for slow motion kicking is to kick more - try holding on to the back of a chair or a wall so your balance does not distract you from extending your kick.