Theres a lot of things that people do that really holds them back from moving up in the ranks. People wonder why they keep losing, why they're stuck in gold or platinum or whatever. Of course theres a million and one different things you can do in starcraft to improve your game, but here are some of the major ones.
1. Your economy sucks.
3 minutes into the game and you're starting to produce your army, and then uh oh. You forgot to build enough workers. You need about 24 workers to fill up your gas mines and minerals effectively. Most high level players don't build their first barracks or gateway till about 13 supply because they don't want to stop producing workers. Having a strong economy means having the potential to produce a large army. Keep your Castle hotkeyed to a number so you can easily produce workers.
2. You don't spend your money.
This is especially a problem with the zerg players because it can be quite a hassle to keep your queen injecting larva on your hatcheries. If you look at the economy tab after the game and your resource collection rate is higher than your average unspent resources, you diffidently need to work on this aspect. Don't just build 2 Barracks and queue up 5 units. If you see you have a long queue going, cancel the last units (Pressing esc while buildings are selected will remove the unit that would be produced the latest) and throw up more production facilities, or expand. Also after expanding and have 2 bases going you'll obviously have 2 times the cash flow coming. So shortly after expanding start making more production facilities. Don't be afraid to make another hatchery if you're zerg.
3. You don't scout.
Oh you're so clever you're going to make a bunch of mutalisks and your opponent is going to be ever so surprised. Too bad he saw your spire going up and started making turrets, or phoenix or whatever. Relying on the element of surprise only gets you so far. Scouting in the early part of the game with a worker can save your ass big time. You can see early spawning pools, 2 gates, 2 barracks or if he is probably proxying. Easiest way to win is to scout and produce the counter to whatever the enemy is making.
4. Supply block.
You're controlling a xel'naga tower and see the enemy coming your way with more forces than you current own. Time to stark cranking out more, but oops, you're supply blocked with nothing on the way, might as well gg right there. As terran you pretty much need an scv constantly making supply depots, as protoss and zerg, as one pylon or overlord gets finished start making another.
These are just a few things, but they really are the largest things out there.