Ideally, what you want is to neutralize either your slicing or hooking tendencies by balancing out the pressure in your right hand, which closes the face, and the left hand, which tends to open the face. If you slice, your right hand probably isn't doing its job correctly and it's gripping the club too lightly. Conversely, I see a lot of players who hook the ball with a left-hand grip pressure that's too light and/or a right hand that's too firm. It should be the other way around if you want to stop hooking it.
The key is to experiment with the correct right- or left-hand grip pressure that helps you hit straighter shots. This may mean equal amounts of pressure in both hands, or it may mean one hand grips the club more firmly than the other. Grip pressure is a great thing to work on because it's simple, and it's also something you can bring to the course.
Justin Klemballa, PGA, teaches at the Jim McLean Golf School at Sunridge Canyon GC in Fountain Hills, Arizona. You can get more information at sunridgecanyongolf.com.