Eat well before you go to the grocery. If you go hungry, then the simple carb processed foods look so much more tempting. As others have said, if you don't have it around the house, you can't eat it.
In line with my first suggestion: keep a stock of tasty and healthy snacks (like Larabars, for example) in the car and office. The first is so if you are in the grocery store and are tempted by junk foods, you can tell yourself you have a good snack waiting for you in the car.
Shop the perimeters of the grocery store. That's where all the 'real' food is. And spend the bulk of your time in the produce section.
Get a good cookbook or two, especially one that has quick-to-make healthy recipes.
Think of yourself as a Mazaratti or other expensive car, not a junky old jalopy. Just as you'd only use the best fuel in that car, you only want the best food in your body.