If you serve backspin, you follow through down, right? And if you serve topspin, you follow through up, right?
WRONG! At least, you shouldn't. Instead, right after contact, try changing the direction of your racket and exaggerate the opposite motion. Don't try to bring the racket to a stop and reverse directions; whip it about in a tight semicircle, making it almost impossible for the opponent to pick up just when you contacted the ball.
Logically, if you always follow through downward on topspin serves, and upward on backspin serves, your opponent will pick up on this, right? Actually, not very often. Since returning serves (and most other table tennis strokes) is done instinctively, the receiver's instinctive reaction to a downward motion is to read backspin, and for an upward motion to read topspin. So in most cases you can get away with this, though you should vary your follow-through just in case.
Note that you normally wouldn't serve pure backspin or topspin in table tennis; you more likely would serve side-backspin or side-topspin. When you serve side-backspin, follow-through up and to the side; when you serve side-topspin, follow through down and to the side. You'll be amazed at the confusion and havoc this will create.