Don't make it easy on your opponents by being predictable. Mix up your serves with depth, pace, position, spin, etc. Same with all your other shots. Note that this still means you have to be consistent in whatever shot you make, don't just hit a wild shot to try to be unpredictable.
The flip side of this is when you know how and where your opponent is going to hit a shot you can set up an easy defense or an attack.