Reconnaissance

Your first recon task is to find the enemy. This is important so you will know what direction attacks will be coming from, and where you need to send attacks of your own. It's also good to know what direction your opponent is going in-- if you see a Protoss player building several Gateways, you can expect a heavy Zealot attack.

Recon Rush
In Allied games, sometimes a team member might send out a early to scout the map, using shared vision to pass that information to the other team members. In one on one games, this can be damaging as even a single worker is very important at that stage in the game to build up, but in allied games you can usually count on your teammates to cover for you. After finding the enemy you can make team decisions as to whom to attack, what person will attack which enemy or, most likely, who you will all concentrate your forces on. Zerg Players have the ability to send out Overlords to scout the map at that early stage in the game, but because Overlords initially travel very slow, it might be faster to send a Drone instead. Keep in mind that this will get that scouting player behind, and they might need to be protected early on until they can recover economically from losing a worker at such an early stage in the game.

Zerg players can park their Overlord over the enemy town fairly early in the game if they happen to find them. This works very well against Protoss because to attack the Overlord they must build Dragoons or a Photon Cannon. You can often sit over their town and watch them for quite a while. Protoss players should use a Dragoon to get rid of the spy as soon as possible. Generally, as soon as the Zerg player sees a finished Cybernetics Core, they should move their Overlord away to high ground or a safe territory. Terran players can use a Marine to get rid of the Overlord so this isn't a concern with them. Zerg players must wait until a Hydralisk is done before they can get rid of the Overlord. If at all possible, move under the Overlord before attacking it to increase the chance of killing it before it gets away. Losing even a single Overlord at that stage of the game will disrupt the Zerg player's buildup strategy.

Standard Recon
More conservative players can wait until their first attack unit (Zergling, Marines, or Zealots) before sending out a recon mission. It is up to you to decide how many you want to send out scouting, and it depends greatly on the map size. On large maps, only one Marine or Zealot will take forever to explore the map and find the enemy unless you get lucky. Most likely the enemy will be well established before you find them. So the larger the map is, the more scouting troops you should probably send.

Keep in mind that if you get attacked by a full force while your troops are out scouting, it could spell the end for you. Scouting is a risky maneuver but it can pay off. Sometimes it's simply a matter of luck. If the enemy gets lucky and finds you early, it can be over.

After you find the enemy you have to make a decision if you want to attack or continue to build up. If you see a definite advantage, attack. Keep in mind though that the enemy may be able to build up a defense in the time it takes you to walk your troops across the map.

Watching the Enemy
It is important to make periodic checks on the enemy town to see what units are available to them, and possibly what strategy they might be trying for. It's important to become familiar with the tech tree for all three species, even if you specialize in playing only one of them. For example, if you find a Protoss player with four Gateways, a Nexus, and nothing else except Pylons, you know that they are probably going for Zealots. If they have one Gateway, a Cybernetics Core, and a Robotics Facility, expect a Reaver drop.

Stopping various strategies is often as simple as catching the enemy while they are attempting them. Examples of this are enemies building Bunkers/Shield Batteries/Barracks/Gateways outside your town, trying to set up defensive formations in high traffic or key locations, or expanding to new resource nodes. As noted above, it's also important to determine what units are available to your opponent, and whether they are focusing on a one-unit strategy or not.

Terrans
For this task, Terrans have the easiest time. They can use the Scanner Sweep ability of the ComSat Station, after building a Barracks and an Academy. This cannot be countered by the enemy without destroying the ComSat Station. Terrans can use Marines initially to scout until the ComSat station is available.

Vultures with the Ion Thrusters upgrade make incredible scouts. Send them around the resource spots using waypoints. You can lay Spider Mines at the resource spots to spot and attack anyone trying to build by them (keep in mind that Mines are not activated by hovering workers.) Mines make incredible scouts-- lay them in well traveled areas, and outside the defensive perimeter of your town to spot and harass enemy troops. An upgraded Vulture can also be used to run right through enemy towns-- their fast speed sometimes allows them to zip right past defenses.

You can also use cloaked Wraiths to scout the enemy if they do not have detection yet. Usually a ComSat is more effective, though, especially if you remember to use it every time its energy reaches full charge.

Protoss
Protoss have a hard time with spying until they are able to use Observers. Use Zealots and Probes to scout the map until Observers are available. Unless you are really doing well, you won't want to leave your expensive combat units like Zealots and Dragoons sitting around resource spots. Once you have enough Probes gathering Minerals and Gas at your main base, send them out to sit and wait until you're ready to start on an expansion town.

Once Observers are available, use them. They make perfect scouts, since they are small, fast (when upgraded), and, of course, invisible. Hide them over the resource spots, send them over the enemy towns, leave them along land routes between your bases, plant them any place that you can. Researching their speed and sight upgrades makes them the best scouts available to any species.

Zerg
The main Zerg scout is the Overlord. Be sure to research the speed and sight upgrades. Overlords are visible, vulnerable, and valuable, so a better scout is...

The lowly Zergling! Place a burrowed Zergling at every resource spot. This will only cost a little of money because they are so cheap. You will find that the computer Zerg AI uses this strategy quite well. You can burrow larger forces as more Minerals are available-- 6-12 Zergling won't impact your economy much, but can quickly destroy and enemy attempt to expand. Place Zergling at other high traffic areas to watch enemy movement. Often your Zergling will be able to sit there untouched for quite a while, giving you a free scout on the map.

This also works with Drones and Hydralisks. Burrow Drones at resource spots until you have enough money to build a Hatchery.

Starving the Enemy
Often winning the game is as simple as making good recon on enemy expansion. It is important to grab multiple resource spots (especially geysers) in order to build more units, or to afford the more expensive units such as Carriers, Battlecruisers and Guardians. If you can deny the enemy these additional spots, they will not be able to fund further troops and you will eventually be able to overpower them if you use your troops effectively.

Many inexperienced players don't realize this, and when they lose the game they wonder why they lost. Often the reason is that their enemy mined more resources than they did. Take a look at the stats after each game, and find out if that was true. If so, you know that it is an area you want to improve at. There are always corners that can be cut to make resource gathering faster. It is up to you to find out what those corners are. While having the most resources doesn't guarantee victory in and of itself, it certainly makes it easier!