some simple tips,
Build orders, you dont have to build the exact same way on every adc each match, make the build order meet your needs for the individual match
Practice last hitting, easiest way to do this is pick annie or nasus in coop or custom
selective poke, if you see one of your creeps are low and can be one shot by the enemy adc for cs, get cloes to the minion and hit the enemy adc for a free hit or two, he will have to choose between cs and trading
freezing lane, a bit of a more advanced technique, but if your slightly winning your lane, let the creeps push to just out of your tower range, the trick is to only last hit and keep the enemy creep wave larger than your own, if the enemy creep wave kills all your creeps, stop infront of the creeps and take a few hits until your wave gets to them. if the enemy comes in for cs, harass and deny him. you will start to see a lead in levels cs and gold by then.