Hold the club tighter with the last three fingers of your left hand to prevent a hook. Conversely, lighten grip pressure in the left hand to prevent a slice.
I had a three-shot lead on the tee of the 72nd hole at the 2002 Senior Tour Championship. There was water on the left side of the fairway and a strong right-to-left wind. I obviously wanted to take that water out of play, so I used a simple technique I learned early in life from my father for preventing hooks and pulled shots.
At address, I make sure to hold on tighter with the last three fingers of my left hand, and I maintain that firmness through impact. It has been a reliable tip for me over the years, and it worked this time.
The ball held safely against the crosswind trying to push it left. I parred the hole and won by two shots over Gil Morgan, who birdied the final hole.
You can influence the flight of the ball through grip pressure. If I want to hit the ball from right to left, I will lighten the grip pressure in my left hand so my right hand takes over at the bottom of the swing to help close the clubface and make the ball draw.
Because many of you slice the ball, you might want to experiment with that latter technique. But you still need to grip firmly enough with the left hand to control the club.