Slice and Dice excels at harassing maneuvers. Slice in, follow up with another skill and then Dice back out to safety.
Cull the Meek drains an enormous amount of life when used in the middle of the fray. You can use this to bait opponents into thinking you are weaker than you really are.
Cooldown reduction is especially good for Renekton, allowing him to both quickly build up and use his Fury.
Pay close attention to Renekton's Fury gauge as that will usually signify when he is about to attack.
Keeping Renekton from being able to fight and gain Fury by continually harassing him will severely reduce the effectiveness of his abilities.