Stance, stance, stance. This is #1. But you can't stand in a good stance if you don't also chill out at the finish of the turns.
To the casual observer, slalom looks like it is all about digging in at the finish of the turn. That is why nearly all recreational skiers ski back foot heavy and can't hold a lean through the wakes. Rather, consider that course slalom skiing is about efficiently getting from side to side.
Try not to focus on leaning so hard and fighting the boat's pull at the finish of the turn/start of the lean. Rather, try to just be ready to establish a leaning posture. As the boat's force comes onto the handle, focus on retaining this leaning posture and simply not letting the boat pull you up out of your lean. The max effort to retain your lean should occur in the zone between the white water and through the first wake. The idea that the turn ends smoothly and the effort to maintain the lean builds through the wake crossing is what is meant by a "progressive" lean. When you do this correctly, the amount you lean away from the boat is more consistent from the start of the lean until you decide to start to let up. You are in control because you built the lean progressively.
Conversely, cranking and digging in at the finish of the turn means you have already used up your power. When the boat comes on the handle, you do not have enough left in the tank to resist the force and stay in you lean. The boat pulls the skier out of the lean and onto a flat ski. This makes the wakes scary.
So chill out at the finish of the turn, reserve some intensity for maintaining the lean through the first wake.