Scouting
The first thing that you should do in a game is scout. I like to send my first probe or scv. Why? Because you get them early enough to see the roots of there strategy. If you are too early however, then you only see where he/she is, not what strategy they are using. Sometimes your enemy is in the last place you look, and that is very dangerous. When you find him its too late to stop him. If you are scouting with a probe, scv, or drone, it won't hurt your economy too much. If you decide to scout early. I doesn’t matter how you scout, but if you don't you'll most likely lose.

Keep an eye out.
You will want to know if your enemy has expanded. Make sure you know the map. If your opponent gets an expansion then he/she will be twice as hard to kill. Send units to the mineral places to keep watch. Zerg are nice because they can burrow. Also, keep an eye on your opponent's base. This is easiest with Terran because they have comstat. If you are not Terran just send some units into your enemy's base, regardless of weather or not they get killed. If he is building powerful units I think you will want to know about it.

Use units with special abilities units.
This is my weak point. I get lazy and say he's dead; I'll just use brute force. I can think of so many games when some Psi storm would have won me the game. Get magic units quickly, for example, Psi storm for Protoss, science vessels and Ghosts for Terran, Queens and Defilers for Zerg. If you rush to get them, however, you might regret it. Just make it a priority. You probably all know how important irradiate and psi storm are. They stop almost everything. Always have spare temps or a vessel around to use them. It's always nice to have Broodling as well. How do you stop Tanks or Templar? Broodling their guts out so they have to replace them like mad. Don't save making them to the last minute or you will be sorry.

4.Tanks/Reavers/Guardians
In most games, these are essential to winning. This is a lot more of a strategy than a tip. Sheer numbers doesn't always do the job. These units are too important to just ignore. They are great because of their range and power. You have to spend a lot of time teching for Guardians so they are a bit less important. The infantry for terran is very weak. If lurkers, reavers, or hydras are dropped in your base, you may be in trouble. To solve this problem have a couple tanks spread in your base along with missile turrets. Tanking an enemy is very important as well. It will stop large armies of zerg or protoss and will kill lurkers and other things. They out range every other unit. It takes two Psi storms to kill a tank. It takes two tank hits to kill a templar. This is very important to know. When large armies come to kill you make sure you get some tanks to hit the mixed in templar. Reavers do MEGA damage and nice splash. If you have a lot of them, Zerg will wince every time he attacks you with ground. Personally if I saw a Protoss player with Reavers, I would be sweating it out. Use ground units for scouting and defending. Keep in mind a Zerg player can easily out number a Protoss player so you need Reavers!

Upgrade
Understand the importance of upgrading. During a very close match, the upgrades will be the deciding factor. If you both have the same amount of units, but you do a slightly larger amount of damage because of upgrades, you'll have a much better change of winning. It takes two hits for a Cannon to kill an unupgraded marine or zergling (ow). If you upgrade, it'll take three hits to kill them. Things are looking better already. One thing that bugs me about upgrading is if you're in a game where you don't have many minerals to spare and if you upgrade that means that you'll get fewer guys, especially in the beginning. Is it worth it? Depends on the game length. Personally, I think it's up to you. But upgrading is very important and it should be done in every game.

Micromanagement
This is what separates the good players from the ordinary. It is early in the game; you and the enemy each have three dragoons. How do you win? You guessed it, micromanagement. Highlight all three of your Goons and hold shift. Right click the enemy dragoons one at a time, and they will each be attacked more efficiently. Note that the shift command is very important. Of course, upgrades will help you win too. If a stray SCV comes to your base, you can use micromanagement to minimize the losses. SCVs are strong. If you don't micromanage, you could lose the drone it's attacking. Get two drones and run the one being attacking around the second drone, and it will survive. Say you are attacking a missile turret with three wraiths. If you move the wraith that is being shot at and about to die out of the turret range, the Turret will automatically attack another wraith. Then you can send the hurt wraith back in and continue with the attack without loosing any of your units. You can do this multiple times if you need too. Micromanaging can be used in a variety of ways. One of my favorites is 'luring'. If you have a huge army in front of your base, run your fastest moving unit close the army and then back to your defense. Some of the enemy’s units will follow. This is fun to do against air units. What I have told you are just some examples. Another example: you are building a wall of cannons. Do you really want to bring your cursor to the build button then click the cannon button and place the cannon where you want it? Very time consuming. Instead put your cursor where you want the cannon to be and press 'b' for build then press 'c' for cannon. This works not only for cannons but other things as well. Micromanaging can be used in many other situations. See the bottom of the protoss strategy for more.

Be aware of cluttering
Cluttering is when you keep all your units really close together. This has its advantages and disadvantages. When you are tanking you should obviously keep your infantry close to your tanks. But what happens if they get in a Disruption Web, Psi Storm, or Plague? Ouch. If you clutter your buildings together it'll be a nice place for your opponent to launch a nuke or plague. If you have a fleet (12) of Reavers in your base, you may think 'oh nice no ground is gunna kill me'. Once again a good psi storm, ensnare, plague, or EMP could get past your cluttered reavers. Maybe you should spread them out.

Go to the next level
Don't stick with one particular strategy for too long. If you decide to use mass Zerglings it may work well for you for a while but eventually they'll figure it out and go "hey! I'll make Firebats". If you are smart you will switch to Hydras and those Bats will get toasted.

Defense
In a well-defended Terran base there should be Tanks scattered throughout, not just at your choke point. Use their range to protect your whole base. If you clutter your Tanks and your enemy drops, it will be easy for him/her to find a place to drop outside of your tank's range. If they're scattered, however, there will be nowhere for the enemy to land without getting hit. If you're not Terran, many people will drop you. When they do you'll want to have lots of movable units to attack them with, and be sure to put some defense by your mineral line. You won't regret it.

Countering
This may seem stupid to some people but it can be helpful in some situations. If you're playing an allied game against an opponent, you may want to consider counter attacking as an option. You're playing a game where everyone has a big army. Your enemy sends his army (and his defense) to kill your ally. Don’t bother saving your ally, KILL YOUR ENEMY. Allie can run for all you care. If you're in the other position, don't depend your on your ally. In most games they suck. If you're attacking, make sure you're prepared incase your victim's ally counter's against you. This is why you should always keep some units behind for defense. Don't ever waste your attacks. Wait until the time is right.

Expand
If you expand then the game will be yours. You will have twice the rate of mineral intake then you had before, and you can support more units. Should you defend a new expansion? It depends. I don't usually guard expansions because I'm lazy. If you're playing with Zerg, Nydus Canals make expanding much easier.

Keep the pressure on
Don't stop attacking your enemy. Force him/her into playing defensively. Keep pushing at the entrance until you see an opening. Eventually the enemy will crack. In my experience the person who's playing in a more offensive style normally wins.