- By the late game stages, you should ideally have a scout at every resource node, and every Xel Naga Watch Tower. Cloaked/burrowed units are especially useful for this purpose. When the enemy goes to expand, you'll know it and be able to promptly take out their expansion before it's established.
- Generally a good combination to attack with is long ranged anti-ground siege units (Siege Tanks/Colossi/Brood Lords) with ranged units that can attack ground and air (Marines/Stalkers/Hydralisks) In the case of Siege Tanks and Marines, the Siege Tanks will pound enemy defenses and pretty much any ground unit nearby, while the Marines protect the Siege Tanks from air units, and any unit that gets to close.
- Practice league heavily favours people who build Reapers, or turtle and mass Void Rays/heavy air unit fleets. Not to mention the game speed is almost unbearably slow.
- Get detectors early, in order to avoid being screwed over by Banshees, Dark Templar, or Roaches.
- YOU MUST CONSTRUCT ADDITIONAL PYLONS. Seriously, keep an eye on your supply count. There's nothing more frustrating than having the resources to amass a huge army, but then realizing you don't have enough Supply Depots/Pylons/Overlords.
- If you're playing as Zerg and you're making Mutalisks, don't attack with them until you have at least 7-8. Once the enemy sees that you have Mutalisks, they'll most likely pour an arseload of resources into anti-air (building turrets by their base). But if you surprise them with a decently sized force, you can do some pretty serious damage before they can react. Mutalisks can even take out enemies that counter them (Hydras/Sentries/Marines) if the Mutalisks outnumber them by a decent amount.
- If you're playing as Zerg, GET INFESTORS. Just about every Terran player has a habit of massing Marines. The Infestor has an ability called Fungal Growth which does 36 damage over time (Marines have 45HP, or 55 if they have the shield upgrade), and it prevents enemies from moving (they can still shoot though). They can also use Neural Parasite (Mind Control), which is useful against single, tough units.
- As other people have said, scouting is of the utmost importance. If you're Terran, get an Orbital Command and some Ravens. If you're Zerg, morph an Overlord into an Overseer, then plop a Changeling into their base (think the Spy from TF2, except it can't attack). and use your Queens to spawn Creep Tumors. If you're Protoss, use observers.