Usually when I teach someone to slip, i make them shadowbox and use a slip line. To make sure you are not bending at the waist hang a bag (or something) from the ceiling above your head. You should be passing directly underneath the bag not in front of it - which happens when you bend at the waist. If you have a partner, it helps if they hold a shield in front of your fron leg, that way if you do bend you end up hitting the shield. There are plenty of ways to detect bending at the waist. It's just a matter of repetition and muscle memory. Be creative - most of all practice correct repetitions.