It takes time. I studies Tae Kwon Do for 5-6 years and never got it completely, but the best way I stretched was doing some initial stretched, warm up for a few minutes for your legs/muscles to get warm and limber, then stretch again. You'll notice quite a difference, but even with this it will take time. Or, if time isn't on your side, you can try the Frank Dux method in Bloodsport.