Chairman Ray:
Starcraft is a game where you have to be on your tip top mentality. If you are losing or tired, force yourself to stop playing. Only play when you are totally pumped. You will probably win 2-3 games in a row. Know your limit though. A lot of people go on a win streak, and then decide to play one more game before the leave, only to lose it and then lose the next 5 games. They lost that first one because they are tired. They lose the next 5 because they are both pissed off and tired.
So whenever you start a session, make sure you are fully ready to play. You will probably win your first game. If you lose your first game, make sure you re-motivate yourself before jumping straight in the next game. When you are nearing the end of a session and you decide to play one more game before packing it in, don't play that one more game.
That's all the advise I have to give.
Brilliant advice! Only one thing to add to that: If you do end up losing your first game, be sure to review it thoroughly so you don't repeat in the mistake and result in another loss. On top of your initial frusteration, continued losses in the same fashion will lead you to go on an even larger losing streak. It may seem obvious but so many people ignore it and the losing streaks they suffer end up seeming ambiguous to them. Hope that helped!