I would never attempt to expand on a missive from Walt, LOL, but the simplest of answers is: Practice, Practice, Practice.
It sounds harsh, but I have my guys flagging from right after chow Saturday morning until dress parade. Then again at least an hour before battle time and the same for Sunday. We used to gather once a week in the past years for two hour practice sessions, but not anymore, they don't need it.
My guys have the entire alphabet and most preconcerted codes down by heart. And can read the message as fast as the sender can send it.
We have found that facing slightly into or away from the wind really helps the process. Keep in mind though, that too large of figure eights tend to look like the "3" motion. Especially with the wind behind you.
"How do you get to Carnagie Hall?"