Sit on a buttefly position (is this how it's called in English?): sit on the floor, bring your feet in front of your body and keep them together sole to sole Get your knees as apart you can. To help, put your hands on your knees and put the weight of your body on them, trying to get them to touch the floor.
http://www.tinajuanfitness.info/exercises/butterflystretch.html
Something a bit more eficient is lie on the floor on your stomach, get your legs and feet on the same position as said before and let the weight of your body pull you down. Don't forget to keep your back straight.
http://www.balletpeeking.com/y002/y002012l.jpg
Stand in a spread eagle position(if you can) or you can have your feet a little more pointed forwards and then go down in a squat pressing your knees outward.
It feels like you are in some weird pre-labor pregnancy class but works. lol