Always ask yourself if what you are about to do brings you closer or further from your goal (while visualizing your goal).
For example, when you feel like pouncing on a jar of Nutella and finish it down to the last spoonful, if you do this small mental exercise, there are 9 chances out of 10 that it will not happen.
This tip works for a slimming goal just as well as for any other choice you make in life).