Learn from your mistakes. Don't be afraid to die. Think about why you died and how you could have avoided it.
You know how many enemies and allies are on the field. Keep tabs on everyone, be where you need to be. If it's night time and you can't see 4 enemies on the minimap, maybe it's time to be cautious.
Know when and how to use skills. If vary the timing of something by a few seconds, will it make the situation better or worse?
Know your own ability and power. Whether you're able to chase down an enemy safely, or survive an attack for long enough to counter. Whether you can save an ally under attack without feeding them another kill. Whether you should keep trying to escape someone chasing you, or turn around and make a last stand so that reinforcements can avenge you.
The basic skills such as skill builds, item builds, last hitting, warding and juking will come quickly with attention. The decision making skills are much harder to refine.