Start out with a very light grip pressure and maintain it all thoughout your swing (especially at the top of the swing and at impact). Swing as fast as it allows you to without needing to tighten your grip.

Doing so will allow your clubhead to swing freely with increased speed.

Warning: It won't feel powerfull but believe me the clubhead speed will go up!.

[The only time that you would need to grip tighter is when your hitting out of the rough.]