If you must eat fast food for lunch, I'd say make it your largest meal, but make it healthy. Get a grilled chicken sandwich (or even just the grilled chicken w/ no bun), don't use mayo, etc. If you need fries, get a small. Diet soda is way better than regulat, but I've heard that your body reacts to nutrisweet in the same way that it does to sugar (basically produces insulin, affects your blood sugar). Also it can cause you to crave other sweets.
I would suggest eating small meals several times a day in addition to a healthy fast food meal (ie. protein shake for breakfast, some yogurt at mid day, a moderate fast food lunch, some fruit for a snack and then some grilled chicken and salad for dinner.) That way, you're eating all day and never really hungry or thinking about food. Also you'll be less likely to binge on your fast food meal if you aren't super hungry when you order!
Juices are healthier, but very caloric! It's so much better to eat an orange than a glass of OJ...still lots of vitamins, but more filling and less calories. We don't really see beverages as calories so people often end up taking in more than they realize because of sugary drinks. You could mix it with seltzer so that it has less calories, but tastes really good to me!
These are just my own tips on dieting...not sure how sound they are, but they have worked for me in the past