For the follow-thru on the serve, I like to think of myself as a samurai sheathing his sword after a telling blow to my opponent. The tip of the racket should go diagonally downward on the opposite side of your body as your dominant hand as if putting a sword back into it’s scabbard. Not only will this improve your form on the follow-thru, but it’ll remind you that you are skilled in the use of your “weapon” and mighty in battle. All of these thoughts tend to make you feel powerful, often taking your serve to another level mentally.
The samurai mentality also works for the backhand (which is good since most of us have weaker backhands, perhaps the spirit of the samurai will make them stronger)! It applies to the backswing, where you pretend that you are unsheathing your sword and unleashing it upon the ball with an upward stroke. You want to stick the tip of the racket back into the scabbard and drop the head, just as the sword would hang in battle, then draw the weapon at the right moment.
Next time you’re on court, be a samurai, and watch your opponent fall to pieces.