Most tennis myths arise either from a misstatement or from an overgeneralization of a correct principle.
Tennis Myth #1: "You should make sure to snap your wrist on your serve."
A wrist snap or whip is an essential part of a serve, but you're much better off not thinking about it. If you try to tell your wrist when to snap, the only thing you're likely to accomplish is eventually injuring your arm. Keeping your wrist loose on your serve will let it whip forward exactly when it's supposed to--as a natural consequence of the forces generated by larger, more powerful parts of your arm and body. Trying to snap your wrist deliberately can put its motion out of sync with those powerful forces, and this can lead to injury. You don't need to think about snapping your wrist, and you definitely should not try to force it to happen.
Tennis Myth #2: "You should always plant your feet to hit a volley."
When you'll be making contact below the top of the net, volleying will be easier if your feet are not moving, but on higher, slower balls, unless you're already quite close to the net, you'll do better by continuing to move forward smoothly as you strike the ball. This will get you closer to the net for your point of contact, and more importantly, it will help keep you from stopping in the midst of hitting the ball. When you stop moving, your body tips forward, and if you're trying to hit a ball while this is happening, you'll tend to pull the ball downward into the net.
Tennis Myth #3: "You should stop to hit an approach shot."
If you have time to stop and set up before you start your swing on an approach shot, you'll be able to execute a bigger swing more cleanly than if you are still moving, but, if you try to stop in the midst of hitting the ball, you'll be off balance, and you're much more likely to commit an error. If you don't have time to stop well before you hit, then keep moving forward as you hit. As you near the ball, you'll usually have time to slow down a little, and this will make it easier to execute your swing.
Tennis Myth #4: "Roll your strings over the ball to produce topspin."
Amazingly enough, one still hears this little gem of goofy advice being given to unsuspecting tennis students. The last thing you want to do is try to rotate your wrist while you're hitting a forehand or backhand, and it's simply not possible to roll your strings over the ball: the ball is on your strings for less than 1/100 second. Trying to roll over the ball will only make you turn your racquet face too much upward or downward, causing an error.
Tennis Myth #5: "Stay down with the ball."
This is more of a misstatement than an outright myth. Pulling up too early on a forehand or backhand is a mistake, so staying down until the right time is good, but you don't want to stay down completely through the entire swing. Except on certain slice shots, including drop shots, you generally want your legs to push upward as you swing. On most swings, you don't want to stay down--you want to be on your way up.
Heard any tennis myths perpetuated lately? Please share them here.