Today I played some 3v3 team games with some Starcraft 2 friends, and found that early aggression is the key to success. The other lesson was if you're NOT going to attack really early, don't sacrifice economy for defenses, because if they DON'T attack, you'll be behind.
One of the prime reasons for this is that you get to combine the power your armies and push against a single point, and your opponents have to struggle to react to it.
I had a terrible time keeping control of the sensors towers in one game, and that allowed the opponents to both know when WE were coming and consolidate their forces. Lesson learned there: if you're going to try for the sensor towers, don't just throw away units - move a good chunk of your forces there (and move them away if you see something coming you can't beat)!