Rope slack in slalom waterskiing is caused when the skier travels faster than the boat. It happens when after the ski crosses the wake the skier continues to pull without putting the ski on edge to slow it down. When the ski is not put on edge after the wake there is minimal resistance to help decelerate. Don't hesitate to roll the ski to the opposite edge. By doing this you set yourself up for the pre-turn. The goal is to keep the rope taut through the deceleration phase prior to making the turn.
Remember to keep your knees and ankles slightly bent and your chest up. Make sure your weight is centered over the ski. This helps the ski turn by itself so you'll be set up with a tight rope for pull across the next wake.