Below is a video of Ron Doerr, a guide from Jupiter. Notice how he keeps everything LEVEL. The entire stroke stays on the same plane. This is VERY difficult to do. When my stroke gets long, I tend not to stay on plane. Ron, like Lefty Kreh, recommends pretending as if your casting elbow is on a shelf, so that the butt of the rod — and the tip — stay on a straight path.
Also, notice how Ron keeps his hauling hand in line with his rod hand to maintain constant tension during the haul. I tend to pull downward as opposed to outward, which means less tension and a shorter haul. Needless to say, there’s always something to work on in fly casting. Everything is a work in progress. Enjoy.