Basically practice your CS, which is getting the very last hit on a minion to get gold.
Using wards whenever possible, I'm not saying buy 50 wards, just get 1 or 2 everytime you go back to base to put in the bushes by your lane. Warding is very important to survival.
Constantly checking the map to see what's going on. This serves two purposes; tracking enemies to see if they are going to a different lane, and making sure your teammates are alright. If you notice an enemy go into the darkness make sure to tell everyone.
I'd say those are the most important 3 things you should learn.
Like what the other guy said, do not be greedy. If an enemy escapes to his side of the map with a really low amount of health, don't feel obligated to chase them unless you are 100% sure you can get the kill AND leave without dying. Getting kills doesn't win the game, clearing objectives does.