You can't win a game on a map you don't know.  To be a good player, get to know as may maps as you can.  Analyze the map, and when it comes time to play you'll do alot better.

It's a good thing Starcraft came with a map editor.  This way you can look each map over and know a few things about them.  You'll know where the start up points are, the resources, potential expansion sites, and you can even think up some sneaky tactics.  The more you know about a map, the better you will do when you play it.

Don't get accustomed to one map.  You can be the best player in the world on Big Game Hunters and lose like a newbie on Lost Temple.  You have to be flexible, and you have to be willing to lose to get better.  If you know your map like you should, any advantages, places to control, etc - You will shut them down.