Here is what I do when I play Single-Player games:
1) I always "pretend" I'm playing a real player. That means same building patterns, forward builders, attack times, etc.
2) First 15:00 practice a lot. I have a routine that includes numbering, hot keys, and waypoints. I am continually trying to get my resources maximized and times trimmed. This is what I practice the most. The reason I do is because I have a hard time winning against good players with a bad spot. Unfortunately I am still a player that with a good spot I win, and with a bad spot I struggle and make mental mistakes because I am not in my rythym. I want to be a player that can win with a bad spot, not panicking and making mistakes at every instance once things are out of my "set in stone pattern".
3) Contests... I mean against myself. I see how fast I can get 200 villas (around 24:00 right now on a Medit map), or how fast I can Bronze, or how big I can boom and still have a 15:00 Bronze (63 is personal best).
4) 7 vs. 1 deathmatches. I set the comp on hardest, and play on all maps and map sizes. It helps me attack 2-3 places at once, and helps me micromanage with a lot going on. It is the ultimate in carnage!
So how and what do you practice?