Answer: On groundstrokes and volleys, grip loosely between strokes, then tighter as you start your swing. If your hand and wrist are getting tired quickly, you're likely to be gripping too tightly between strokes. If your racquet is tilting unintendedly, you probably need to grip tighter as you swing.
On serves, you'll grip loosely through most of the swing, but part of the proper swinging motion is your fingers pulling the racquet into your palm as you strike up at the ball. This will naturally tighten your grip enough before impact to ensure that you hang onto the racquet and have directional control of your serve.
