That is SO FUNNY coz I used to do that in high school!!! In fact, it was a big thing then to try and do them.
So, I can do the front splits and I acheived it by everyday just stretching for about 15 minutes - especially the inner thighs. I would sit with feet soles touching and pull feet as close to body as possible, or sit with legs in V shape and lean head down on one knee and then on the other, or would sit in lotus position, stand with legs apart and try to touch head on floor and then - when warmed would just keep inching down day after day until one day - TA DAH!!! - I did the splits.
I can still do it today - even without warming up!! Its seems that once you do it a few times you can always do it. of course I can get down with more ease if I stretch and practise but I can still do the splits. Though, never did master the side splits