Keep Your Eyes Straight Ahead
Mechanism / Why It Happens
Look in the direction of the grind while locking in, and you'll end up in a lipslide.
Since you're going to grind along the coping, it may seem natural to look down the coping as you lock in, but this is actually a common trap. If you do this, your shoulders will follow your eyes and rotate, that rotation will transfer to your feet, and the board will rotate as well. As a result, your rear truck will come off the coping.

What to Do
Keep looking straight ahead until you are fully locked in.
Approach while looking at the exact spot where you want to lock your board in. Even after you start