Hotkey the Comsat Station to 0. This way you can get a Sensor Sweep quickly without having to click on your Comsat Station. You can then get a Sensor Sweep by pressing 0 and then S. Getting a Sensor Sweep can be critical if your units are under attack by cloaked units.

 For defense, build Bunkers (filled with marines) and Missile Turrets together and place Siege Tanks beside them. This combination provides a great defense that's hard to penetrate.

 When both Comsat Stations and Yamato Guns become available, use them together to take advantage of the Yamato's superior range. For example, when you want to take out a Missile Turret, sweep the Missile Turret area with a Sensor Sweep and then target it with a Yamato Strike. The Battlecruiser will move in to take it's shot but it will still be outside the Missile Turret's range.

 Build a Factory or Barracks behind the enemy's position and start cranking out units for a backside attack. If you can't afford to build there, or can't get an SCV to that position, fly an existing structure into position and then begin building.

 Use the Science Vessel's Irradiate ability on biological units. This can be very effective, even against High Templars or Defilers.

 Use bunkers to prevent enemy movement. Place them at any choke point the enemy must squeeze through to pass. Before it is destroyed, a carefully placed bunker can do huge amounts of damage to the enemy.

 Use bunkers in a leapfrog tactic, building them one after another, even deeper into enemy territory, with the previous bunker protecting the one under construction. This method is particularly useful against the Zerg.

 Build lots of Firebats. The Zerg are an organic race with mostly ground-based attacks. Firebats are always a cost-effective unit to use against Zerg ground minions.

 Use Battlecruisers more for defense than offense, especially in multiplayer games. This is because Zerg Scourges can take out a Battlecruiser for half the cost. In a resource battle you'd lose.

 If a unit becomes infected with a Parasite, the enemy can see verything it sees, so use the infected unit to fight the enemy. Never keep it around your base.

 Take out enemy Gaurdians at all costs. A group of Gaurdians can reduce your base to rubble very quickly. Keep Wraiths or Battlecruisers around for this purpose.

 Whatever you do, don't leave an outpost Command Center for the Zerg to infest. If this happens, you'll quickly have Infested Terrans blowing up around you and that isn't pretty.

 When attacking a Zerg base, always take out the underlying tech-tree buildings first. You may be tempted to go for the Hatcheries, but you can cripple your opponent more by taking out the structures necessary for building advanced units. Take Zerg structures out in this (or any) order:Spire, Ultralisk Cavern, Hydralisk Den, Spawning Pool, and Queen's Nest.

 Again, Firebats are important, but only against Zealots, so don't go crazy building them. Make just enough to keep Zealot groups at bay.

 Build ghosts for defense. They're a little too expensive to produce in bulk for offense. However, when an enemy Carrier or Reaver comes by, you can use the Lockdown ability to neutralize them.

 Use Science Vessels. Their EMP Shockwave ability is especially effective against the Protoss (it removes their shields). The EMP Shockwave is as effective for attack as it is for defense.

 If the Protoss are employing Arbiters, use a Yamato Gun to take them out quickly and reveal cloaked units.

 Watch out for Observers. They cloak and will locate your own cloaked units. Use a Comsat Sensor Sweep to check for Observers in areas where you plan to use cloaked units.

 Keep Missile Turrets around your entire base as an early warning system. If this technique prevents just one Shuttle from dropping a pair of Reavers, it's worth it.