There's a few ways I go about creating.
1) I see a routine I like, and work at it until I can perform it in a way that is my style. Usually this involves rewriting the script from the ground up, often it involves changing the method a bit. It also frequently means learning several different versions and combining different methods to find the one that works best for me. An example of that is my routine which I call "A Psychic Game" - I took Spidey's "Out of My Mind" and Hector Chadwick's "Reds and Blacks" and smushed the methods together. Then I made a couple more minor alterations to suit my own style. Now it's a routine that's pretty solid, and also very "me".
2) I come up with an experience I want to give the audience, and figure out how to make it happen.
I think most of my really good material has come from this method. A really common way I go about this is thinking about legends and myths, and trying to recreate them. Or I'll take historic accounts of strange and unexplained things, and find a method to pull it off. Or I'll find a way to explore a common human condition.
3) Evolution. Nothing I've been performing for a long time is the same as when I started performing it. Over the years the scripts morph, the methods improve, etc. Eventually you realize you have a totally different thing than when you started.
My biggest piece of advice is this: Find a problem, and solve it. Then solve it again, and again, in different ways.
Creating just to create is a good exercise in problem solving, but it usually results in mediocre material. Write those ideas down and then pick one to develop for real.