I get a lot of questions that ask what the "#1 thing" for this or that. Well, I will tell you that in knapping, there is always a combination of things going on. That's what makes it so difficult.
All things being equal, high quality glass is the best material for flake travel. High quality obsidian is a close second. Just look at what was done with obsidian in Meso-america. The long blades are marvels of the ancient world. If you want to learn to run long flakes, practice on glass.
Anyway, here's the deal, to run long flakes consistently you've got to have six (6) things: (1) experience, (2) well prepared platforms, (3) lots of force, (4) a good grip or support for the workpiece, (5) even support for the flake or no support at all, and (6) a non-stepped, convex surface with one or more ridges.
(1) You get experience from many hours of failures.
(2) You learn to prepare platforms from many hours of crushed edges.
(3) You learn to apply large amounts of force without breaking the workpiece by breaking many workpieces...
...so, you get where I'm going with this?
Your flintknapping will improve slowly by correcting mistakes and performing damage control. If you absolutely have no idea how to correct the mistakes, then someone must teach you in person.
Hope that helps.
Edit: Oh yeah, get yourself about 20 bottle bottoms. You can't expect good results on anything less than 20 pieces of material, at a minimum.