first, you should do wide lunges, and reach to the floor, then, if you have someone else on hand, the best way to do it (how we do it at gymnastics) is to kneel in front of them, and wrap your arms around their legs, then lift your leg up for them to hold on to and they should pull that leg towards your body until it is slightly painful. you should hold that for about 30seconds - 1 minute, and then your 'helper' should slowly put your leg to the floor in front of you and the splits will be better!!! remember that the front leg (the one that is up) should be straight!!
another good way is to lie on the floor, and get someone (or something) to hold your back leg down. then raise your front leg and ask someone to pull it down towards your head, until it is painful. hold it there for 10 seconds, then push against their hold for 10 seconds, matching their force so as they don't let go, and then allow them to pull your leg slightly further for another 10 seconds. remember that both legs need to be straight in this one and your body cant be twisted, you have to lie in a straight line, else your splits could end up twisted, and keep your toes pointed!!! =)