As Crackerb0x said, you don't use Zone of Control on just anybody in any place. You use it on things that are already kinda slow to bring them to a crawl(or things that need every inch of their speed to accomplish their mission). Use it in places where swinging wide forces them to deal with blocking or hindering terrain they don't want to deal with. Try using multiple ZoC units to create large overlapping zones. (The rules explicitly say that Zone of Control is most useful when you have the advantage in terrain and/or numbers.) Remember that your force is a team, and have buddies covering your back. A nasty trick might be to combine ZoC with HT units, so you can cook someone to slow them, then Zone of Control to lock them down completely and make them an easy target.
To me, Zone of Control is best suited to breakthrough or hold the line games, where you can force defenders to withdraw before your advance, or stand firm and make attackers break after crashing into your wall, respectively.