One of the easiest "drills" for training a rider who flails or rides with the "mystery date" (rear hand sticking straight out over the toe edge of the board) is to ride with your hands holding onto the hem of your jacket or pant leg. Do not let go unless you are going to eat shit and need to regain balance and then get right back to hanging on.
Use of upper body is a huge problem and a terrible habit that new riders get into to and the "mystery date" is a huge impediment to good toeside turn initiation.
In freestyle and ultra dynamic riding, upper/lower body separation is needed but don`t mistake this for leading with the upper body which is not the best way to ride.
+1 for on the bottom of the jacket.
A more extreme methodology a co-worker showed me was to take your forward hand and put in on you rear shoulder(left hand on your right shoulder if your regular and vice-versa), and your rear hand behind your back.
With this weird positioning of the arms, you feel more stress in the upper body when you use it, thus making yourself very aware when you do. You should only use this as a "self check" and not as a full on riding style.
WARNING!!! if you eat shit backwards, holding your hands behind your back is a fantastic way to dislocated your shoulder. I rider very comfortably with my hands back, but the second I think im losing balance they go right back in front. So just be aware.