When you have a day off from your weight training. You should spend that day thinking about your diet and being extremely strict on these days. Follow a eating schedule and plan on the rest of the weeks diet. This way when the week resumes you are less likely to fallout from your diet.
If you use a diet plan throughout the week you will run unto less times of the day when you have to resort to unhealthy food to appease your hunger.
Mental strength and physical strength go hand and hand. I find that the more mental strength i have the easier it is to push myself to the max in the gym and get those gains. So by practicing the self control of dieting you have increased your gains in the gym and can advance your level of fitness.
Constant self evaluation and examination will help you stay on target. You may not recognize you are loosening up in the gym or diet. On your days off it is really something to think about. Be resolute, want it!