The best way of trading is standing in a bush and waiting for the enemy to walk by. Load your Q as long as possible because it deals 300% damage after 2 Seconds! If it isn't possible to use your Q this way, try to engage with your R and instantly load Q after you hit the enemy. If you aren't behind, it shouldn't be a problem to kill the enemy after that combo :D If it will be close, don't be afraid of use your ignite :P
In Late Game the Cdr on your R is around 70 seconds, so you can use it as often as you have it ready.

Farming: Try to keep the health of the minions at the same level, so you can use your W to last hit them.

Have fun :D and excuse me for my english :'D I'm sorry.