Men's Health magazine has a great feature every month called 'eat this, not that' which shows how small item choices can make a difference. One thing I was shocked about was the Bacon Egg and Cheese English muffin at Dunkin Donughts is actually better for you than the Whole Grain bagel with low fat cream cheese. The former has fewer fat, carbs, and calories. The latter is just marketing.
Even if you order a Big Mac and fries and only eat half of it, you've still cut your caloric/fat intake by 50%.
The “Eat This, Not That” book for the supermarket is my bible. My family wants to burn it.