I use a stretch I like to call the crane's stretch. It has gotten my roundhouse up to 7'3"(though I am 6'2, lol). Plus it allows me to apply a full force sidekick to face level, without having to chamber my body back, giving me a far more stable base. From relaxed stance, lift one knee and grab the middle of your foot. Then keeping your body upright, extend your leg upward to the side. Hold for 30-60 secondsand do it several times a day. Try to bring it above your head and make sure the leg is fully extended. You will see a drastic improvement to both the height, and the speed of your round kicks.