Assassin*
BaronReplays
Watch replays, watch for cooldowns, watch for rotations. See if you can pause replays and know what will happen in the future/ how to set up for it. Consider every possibility at every moment in the replay. Look for the items of champions each time someone backs to consider fight advantages. Always be there to farm the lane as jungler if the minions will die to a tower.
Know your damage, as such, don't play too many champions so you are more comfortable knowing your damage. Consider abilities and their ranges/cooldowns, especially cc abilities when playing an assassin.
Deny people of farm. 30ish cs = a kill. Did you see an enemy flash, camp that lane and help push the wave after they die, farm, then go back to that lane. ~30 seconds to cross half the map (at least the last time I checked), and flash has a 4-5 minute cooldown depending on masteries. Pink ward the lane you want to gank, come back 1 minute later if they saw you clear their ward. Also, try to watch for what the enemy jungler is good/ bad at and abuse those facts as well as keeping track of where they go. If you can't focus on these things in game, try to focus on it in replays... the repetition will make it easier in game.
I don't do any of this, because I can't motivate myself to try to get better...