After playing over 50 games in a day, i could feel my wrist was tired like never before (and i have played games for longer than that in the past). That's a good sign. Those muscles are going to develop like any other muscle and with my good diet and all the other stuff i do (working out, sleeping well etc.) progress is unavoidable.
"Steady and persistent practice seems to be the only way." - Moon
I also found great analogy to StarCraft mechanics and improving at the game:
Think about your password, one that you're using for very long time. It may be your Gmail account password, or password to acces your profile on your computer. Now go back to when you came up with the idea for it and typed it the first time. You type it carefully, not to make a mistake, letter by letter.
Now, few days after that you type it a little faster but you still have to think a while, not to mess up the order of the letters, and you may type it incorrectly if you're trying to do it faster.
Now, three months after that, you type it fast without even thinking, it became muscle memory.
And finally after a year or more, you type the password with blazing speed and if someone was looking at the keyboard while you were typing, he couldn't even tell how many letters the password has, you're so fast.
The interesting thing is, that you didn't go to PasswordTyping forum, to find ways of fast typing your password and getting it correctly every time. You didn't play minigames to increase your speed of typing each letter, you didn't buy special PasswordTypingPro keyboard. You just did it over, and over, and over again.