Every so often, go Overlord hunting. Air units are ideal for this job. They are almost all fast, can fly across water, and usually have strong anti-air attacks. Wraiths (can also cloak), Scouts, Mutalisks, and Valkyries are highly recommended. Destroying even one Overlord can greatly hinder your enemy in unit production, and will force him or her to morph another Overlord. This could delay an invasion for several minutes while you bolster your own forces.

3

Early in the game, try distracting your opponent by harassing his base with a worker. If your opponent only has workers produced, his only choice is to use at least two workers to kill your worker. Don't let your worker die, retreat. Before the two or more workers start gathering resources again, attack with your worker. If the enemy does not respond, build something in his base, but don't finish it, because that would just waste resources. Instead, if his workers attack your building or your worker, cancel the building right before it dies. Therefore, you regain the resources you spent and caused your opponent to lose income by distracting several workers at the cost of only one for you. As a bonus, your worker gathers intelligence about your opponent's base.