Just relax on the beach. When I start really thinking hard about it and trying to focus on distance, and putting some extra muscle into it, that's when I tighten up and start screwing up. With more time and just relaxing, you'll start to better feel the rod loading and your timing and stroke should be easier to fine tune. That said, each time I hit the beach, I start out just fishing and relaxing, but at some point, when I'm tired of focusing on the fishing, I just start trying to bomb out some distance casts just to see how far I can get. That's when I hit myself in the head, or get a wind knot. But each time I get a little better, and then when I go back to relaxing and fishing, my casting is just a tiny bit improved.
Another perspective . . . sometimes it just ain't happening. I don't know about the rest of you, but I certainly have some days that are better than others.