Concentrate on economy and build up during the early game above all, but always keep economy in the back of your mind until the end. Resources are your life in StarCraft. There are two important economic theories in RTS games. I call these two important aspects of RTS gaming Vooch Theory and MayGodHelpU Theory.

Vooch, an old friend and WarCraft 2 hoarding monster, proved to me many times that hoarding money gives incredible power in RTS games. If you are richer, you will have a much easier time taking out the opposition. In FFA's hoarding is excellent! Once they become rich, hoarders throw men at the enemy without thinking much of the outcome in order to keep expansions going while their sheer numbers do damage to the enemy. Note: in War2 this is much easier to do than in StarCraft because of stronger defenses in StarCraft.

Another way to approach your economy is to think of men and buildings in terms of resources. This is something MayGodHelpU, another friend and WarCraft 2 micromanagement master, made clear to me. If you keep this in mind, you won't do inefficient things like waste several precious men saving a cheap building, like a farm or tower. The mircromanagement player considers each man precious while keeping in mind when to sacrifice a few men or buildings to kill several of the enemy. Note: this does not at all imply being defensive, although sometimes you can kill more of them on defense. Do whatever works. Try to climb the tech-tree quickly and get better men so your micromanagement is even more effective.

Understand and apply these theories! The hardest part is knowing when to concentrate on which since these two theories sometimes oppose each other. My style of play tries to mesh these two theories while being ultra efficient in the early build up.

Usually try to get about 20-25 peons per first base, less if there are few mineral crystals. The exact number of peons can be determined by a few factors. For maximum efficiency (if the minerals are relatively close to your hall i.e. command center/nexus/hatchery) build nearly twice the number of peons as mineral crystals plus the number of peons on gas plus the number of peons scouting and building buildings. If minerals are further away you will need more peons. Don't forget to fetch SCVs when they are done making buildings. For Protoss use one or two probes to quickly build a few buildings and send them back to minerals quickly thereafter. For Zerg remember to replace drones when they are "used up" to make a new building.

Get the same number of workers/peons for your second base if you can. Your third base, if you get it early, shouldn't have as many peons. This is because you will be using up too much supply (space available for units made by Supply Depots, Overlords, Pylons) for peons and not fighters, and you will being going way too hard for economy and will be weak for much too long a period during the game. Usually just build enough peons for gas at this 3rd mine, and when the "first base" peons are done walk them over to 3rd base for minerals.

When mining gas, only put 4 guys into the refinery, unless it is far away. Occasionally you'll get lucky and three workers will do the trick. Just watch and see when they only wait in line one at a time at the gas entrance. If more than one is waiting (they overlap for a bit) you are wasting peons. You will not get gas faster if you put too many peons on the gas mine, but it's better to have one too many peons than not enough. Never use gas workers to build buildings. When you expand, pay attention to gas unless it is much higher than your mineral reserve.