When you get engaged by a melee champion place a trap under him and move back after half a second. Then you will be out of his range when he's trapped and you can shoot the **** out of him.
If you move back too soon the trap won't go off yet, if you place the trap before he is near you he will see it and dodge it.
In early laning phase try to poke a lot. Fire some basic attacks at your enemy when he tries to farm (you have a range advantage) and when he moves back fire a piltover after him (even bigger range). Then if he is low enough and you are level 6 you can fire your ultimate too to finish the job without getting in danger.
Don't forget Caitlyn does 5.88/6.67/7.69 % extra damage firing from a brush!