A few thoughts, as I slowly try to improve. These aren't aimed at anyone, and to be honest it's been mentioned earlier in the thread, so i'm just going to reiterate what I've learned:
Don't wander solo after the first teamfight unless there are wards up and you pay attention to the minimap to get out before they gank you.
If your teammate in Hellborne top or Legion bot goes to pull creeps, do not help them - theyr'e getting free exp, you're soaking up extra exp from the lane, and it's just good.
If everyone has their headset, and they can speak, use them. If you don't, then you're stuck with typing and pinging. It can be done without, but verbal communcation really kicks ass in this game.
If you're ranged in a two-melee lane, harass them. I'm horrible at this, yet it gives splendid opportunities to make them waste cash on healing items, and gives you a better opportunity to gank.
Carry a teleport at all times
ALL TIMES, carry a teleport.
Repeat the two above items a few minutes. I'm guilty of not having one on occasion, but having one is very important.