i actually play terran, but i think you can apply it to protoss too. so what i try to do is to always attack in multiple spots. keep your opponent on his toes and make him have to look at multiple spots at the same time.
For example i drop the main and the third, and at the same time, attack with my main army or something. works great against protoss deathballs :D .
you could do things like proxy pylon zealot warpins, warp prism harass, phoenix harass, etc. just do fun stuff :D