Disco dancing

This is a very effective way of killing off melee attackers, and can give you an incredible edge against for instance Zealots:

When attacking Zealots or other sufficiently slow melee troops, divide your troops up into two groups. The player controlling the zealots will likely attack one of the groups with all his forces. When this happens, you have that group flee, while still staying in range of your other group of troops. This way the group fleeing will stay out of range of the melee troops, while they are being slaughtered mercilessly by the group not being attacked. If your opponent goes after the "dangerous" group mid-battle, just reverse the roles of the groups. Voila - dead melee units!

This strategy works especially well with Terran foot soldiers, Stalkers and Hydras against Zealots.

Note that if Zealots have upgraded to use the dash ability (making them faster) this strategy will not work anymore.