Team up if you're playing 2vs2,
One of you go Land, one go Air & focus on 1 unit type (maybe get a few complimentary units but not much) & attack as soon as you've got 4-5 of your unit type.
Myself & a friend have been winning with this tactic lots, he plays Protoss & heads for Void Rays, I play Terran and go for Siege Tanks (I'll also build 3-4 Marauders & 1-2 marines). Once we've both got 5 we attack.
Attack together and in the same location, make sure you attack anything that can shoot back first! Next attack anything that can build something that can shoot back!
The rest should follow...
In 2 vs 2 you'll meet a lot of people playing who are just landed with a random ally... you will paste them