As zilean, After your bot lane has moved to the mid lane, rotate to the bot side of the map or somewhere else on the map so you can continue to farm and gain XP. 

Try and siege objectives with your team during the mid-game. You have strong poking tools which will allow you to siege objectives with ease. Try to poke the enemy down as much as possible while your team tries to take towers (or the Dragon or Baron).

Delay team fights for as long as possible while you harass the enemy and poke them down with your Q. As zilean, After your bot track has moved halfway, turn to the bot side of the map or somewhere on the map so you can continue to grow and win XP. Try to sit goals with your team in the middle of the game. You have strong pose tools that will allow you to sit goals with ease. Try to bring down the enemy as much as possible while your team tries to take turns (or the Dragon or Baron).