Best posts made by Hell

Still great advice! I would only add one point which is, find like minded people to play with. If you are looking to improve on Ladder (TMM now) or Custom x vs x games and find that you are unable to perform good enough to win on your own, make some friends and queue together because at the end of the day, even if you do everything perfectly, you are playing a team game.
As Picard once said: "It is possible to commit no mistakes and still lose." Sometimes your opponent is just better. Use those times to learn from them.