When you're playing a game, always be building units. Don't forget. I had a couple of friends today play with me and they seriously didn't build anything, when the opponents came and attacked us they had like a few cannons so they hit me and we lost. Cool huh! Don't turtle up unless you're going to pool a zerg for mutalisk!
Anyway, TL;DR Keep a nice army up whether it's to attack, or to defend. Especially in team matches!