I got the first 5 mins down pat, then 10, then gradually better in the macro phase. If you make a mistake in the first 5 mins it will likely cost you the game. After the macro phase starts for me (a Zerg player) I found that every lost moment adds up and can be very difficult to catch up there after.

Also, learning to scout and know what your opponent is trying to do. Another critical skill, especially for Zerg.