durning a game people always click attack at there destination (probably an enemy, works best if they use ground units) try this, put a bunch of floating engineering bays in front of you defense -about 4 or 5- when they attack with, hydras or drags, marines etc, they will attack the engineering bay,instead of your tanks, this tactic is use best for human games.

CreepColony's Opinion: I think the units WILL attack the tanks instead of the engineering bay. But you can still hide your defense with them.