Starcraft II is a mathematical game, and numbers do not know what luck is. Everyone starts out in the game equal: At this point in the game everyone has as much of a chance to win as the next person. It is from here on out, believe it or not, that you either lose or win your game. The enemy does what he does but it is completely up to you and your actions that will dictate the game. Your choice of scouting and expanding: Your choice where to take the naga towers or to destroy those rocks, etc etc. Every action you do take uses up something, whether it be resources, troops, or time, each and every one of these is very important so do not waste any. The person with the most time always wins!
The point of this guide is to try and teach retreat. The only other game I have ever seen it build in to the mechanics of the game is company of heroes and if you have played that game you will know what i am talking about. Let's say you are pushing MM an enemy and your able to kill part of his forces - the rest run off. You see a natural and you destroy it. You have a choice here that 90% of people go wrong. You can either charge up the ramp to try and finish his army and base off or you can go home. From whenever I play I see time and time again those reamaining troops try and fly up those ramps and get slaughtered or pitter against a force they cant win and are left to die. But for what?? For another hydra kill and some damage on a queen and units??? Its not worth it. You need to play this game with numbers in mind. If i send in 1k worth or marines up that ramp will i be able to kill more then 1k or will i only killl 300 worth and get slaughtered?
The point i am trying to make is that try retreating and sending your troops back and taking the chance to expand but never send your troops on a suicide rush to "try" to kill em or hope to get "lucky". Call em back, heal em up, and add em to your next wave.