Standing splits are a good way to improve flexibility, and lengthen hamstrings and hip flexors. The key here is not to focus on how high you can lift the back leg. Rather, it's more important to focus on making sure the standing leg kneecap stays facing forward to protect the knee joint, and to be sure that the lifted leg maintains an internal rotation. (In this case, an internal rotation just means trying not to let the standing leg hip open up.) When you get to your fullest expression of the pose, breathe five to seven deep breaths and then slowly release and try the other side. You might find that one side is tighter than the other- don't worry too much about it, just do what you can and if you do it often enough, you'll find yourself improving. Hope this helps!
PS- More yoga tips.