Try this twice a day (ie morning and night), a warm bath first can also help...
Sit and put your legs in pike (both together straight out in front of you), keep them straight and point your toes.
Try and get your nose as close to your knees as you can and hold for 30 seconds.
Next, flex your feet, keeping your nose as far down as possible.
Then sit in a wide straddle, and try to get your elbows on the floor. Hold it for 30 seconds, then reach your hands as far out as possible and try to put your nose on the floor. Hold for 30 seconds.
Then place one leg in front, and lunge from your knee; your rear knee should be on the floor with your front foot in front. Hold 30 seconds.
Sit back, straighten your front leg, and try to place your nose on your knees.
Then slide as far into the split as you are able. Repeat on the other side. Hold each position 30 seconds.
Take it slowly, don't try and push yourself or bounce into it, you need to increase your natural limit but if it hurts to much then stop.