Here are a few tips and strategies that are helpful when playing the game. I'll post more when I can think of some..
Always have detection where ever you go! Including around your base, in your base, when attacking, or even when expanding!
Don't trust no one! B.net is full of backstabbers of now *including me.. sometimes =/ * Be prepared for anything!
Don't constantly mass out on the same unit if you know it's not going to work. That's called common sense. For example, if your opponent keeps going corsair and carriers, don't always go mutalisks, because corsairs RAPE mutalisks!
Be sure to communicate with your teammates on ally chat, unless playing with computers.
Upgrade! If you are going zealots/dragoon combo, be sure to upgrade everything! Speed, long range attacks, everything! Unless you are just going ground, then you probably won't need to upgrade air, unless you are pretty sure you're going to need a new tactic early on.
Storm > Zerg If your protoss, always get storm, especially against Zerg.
Use spell combinations. For example, use EMP against protoss buildings, and then matrix defense a ghost and let it nuke it. Or with zerg, use plague on the cannons, then darkswarm them and send zerglings in.