We coaches here talk about this a lot. What it really comes down to is just stretching, often.
While in your stretch, activate muscles in the stretch by squee-ng them, for roughly 30 seconds, and then relaxing. You should feel a movement in the stretched area, effectively increasing your range of motion.
Passive and active flexibility are different. Pasive (splits) helps you move into active (split leap). Over splits(stretching) increase your range (how "flexible" you are). Being able to activate muscles in over stretches increases your ability to actively use that stretched position.
Hope that helps, and I'm sure other coaches will chime in. I love this question.
For my splits, oversplits works the best for making them better. I also try to hold side stretches and pancake for as long as I possibly can, stretched all the way down.
I was blessed with shoulder flexibility though. I'm double jointed. haha
I TRY to stretch at home every day we don't have practice, but now that i am in school that is kinda hard. Before you stretch warm-up your muscles. Then, (i have 2 panel mats at my house) I put my front foot up on a mat, and hold that for a minute, then i switch it up and put my back leg up on the mat and put my front foot on a paper plate so it slides out for a minute, then i put my front foot and my back leg up on a mat for a minute then slide down to the floor for 30 seconds (about). I don't usually end up holding all of these for a minute, though! And before i do all this, i do the basic pike, straddle, and butterfly stretches, and do bridges when i finish all 3 splits.
Listen to music while you stretch. It can be relaxing or exciting, just make sure it's not scary- you don't want your brain to associate stretching with fear! You can also hold splits while you watch TV and keep your legs straight when you bend to pick something up.
Also, make use of your flexibility during strength training. Hold you legs straight and up in the air doing crunches and whatnot. There's not much point in being flexible if you can't use it for your skills