I suggest just stretching a lot, and trying to do the splits a lot. There's not a lot that can be done I don't think, but I've never had that issue since I was born really flexible. Sorry!
Stretch for about 2 hours a day
Developmental stretching is the method used, stretch to the point of tension hold it for 30seconds then your muscle will stretch and you will find that you will be able to stretch a little further and repeat doing the same thing
You can try to do some splits, even though you can't do a perfect one. Try to stretch your legs a little bit more everyday. Try to do some chores faster so that you can gain more exercising time.
I do ballet and have done so since I was about four but compared to most of the girls in my class, I am not flexible (I can do all the normal stuff like touch my hands flat on the floor from standing and put my head on my knees) but I can't get my leg up very high - and I'm way off being able to do the splits.
Are there any specific exercises that would help me to become more flexible?