Play more Pop'n! Pick some songs like Enka Remix and stuff with lots of jackhammers and chord streams. The more you play the more you'll get used to it. I know I used to have the same problem. I'm sure there's some workout that you can do to improve arm strength for stuff like that but honestly I'm fat and lazy and playing Pop'n did the trick for me.
so is anybody still actively playing pop'n? i kind of want somebody to share scores with and compete. i stopped playing for 2 years and now im playing again since i bought a pop'n cab. I can pretty much clear anything 38 and under, and can clear some 39+ songs