This is the portion of your diet where you have the most control. Anyone can have a weak moment, and eat more than they intended or foods you know you shouldn't have had, but if they're not in your cubbard, it's much more difficult. Few binge urges last long enough for a grocery trip. When you're shopping for food you should be ABSOLUTELY STRICT.
The first thing you'll notice is that nearly all the stuff that's good for you is around the outside of the store: fruits, vegetables, meat, cheese and such, while the bad junk is all in the middle, sugar, flour, potato chips, corn syrup, hydrogenated vegetable oils, etc. Anytime you find yourself going down the aisle for something, remember that you're in enemy territory and watch out!

Here are some things you should buy:
Fish - Any kind, pretty much every diet ever written agrees that fish is good for you, so stock up! Avoid breading though, and remember that tartar sauce usually has plenty of sugar in it, so stay away.
Poultry - Chicken and turkey, these meats are lowfat and have lots of protein. Avoid the cranberry sauce though, it's made with a ton of sugar! If you really crave it, you can make your own using sugar substitute.
Beef - Stick to lean cuts as much as possible, red meat is good for you, but don't overdose on the saturated fat. For hamburger I usually buy 7%, I've found that if it has less fat than that, it's rather difficult to cook. A good trick is to throw it in the strainer after browning, and run just a bit of water over it, this tends to wash off surface grease and give you results similar to a leaner version.
Pork - Again, stick to lean cuts, and watch out for surprise ingredients like brown sugar. It's ok to eat bacon now and then, but use moderation, that's definitely not an everyday food.
Vegetables - Pretty much everything except potatoes and corn are great for you, buy as much of this as you can, in fact I make sure to start and end my grocery trip in the vegetable section. Sweet potatoes make a good substitute for regular potatoes. Some people also like yams (and some restaurants don't know the difference) which have a dark orange interior.
Fruit - I dare you to get fat on fruit, I mean seriously try it, can't be done. Although they have fructose, a natural form of sugar, humans are very well adapted to it, and you don't have anything to fear from fruit. They're low in calories and high on flavor, make sure to stock up. Be careful if you buy canned or frozen though, they almost always add sugar.
Nuts - Nuts make a good snack food, though you need to watch out since they're fairly calorie dense. Also some of the flavored kinds have sugar added, so look for the natural ones.
Grains - Oats and rice are ok, but avoid pretty much everything else. (flax seeds, quinoa, and whole barley while rare, aren't too bad either) Anything that has white flour is out! If you have any confusion about it, think like you're living in the 1800's, ingredients should be short, simple, and disgustingly healthy sounding.

What to avoid:
Boxed items - If it comes in a cardboard box it's just about guaranteed to be bad for you. If for some reason you find yourself contemplating purchasing such an item, read the label extremely carefully. Seriously, you could almost make a diet just off this one idea.
Items with hard to pronounce ingredients - If you can't say it, or don't know what it is, you shouldn't be eating it.
Soybean oil - Tons of things from regular mayonnaise to snack crackers are made with soybean oil. Soy in general isn't very good for you, and soybean oil is about the worst way to eat it. Look for a product made with Canola or Olive oil instead. Remember to stay away from ?partially hydrogenated vegetable oils? as well, they're full of trans fats.
Sugar - Sugar, corn syrup, evaporated cane juice, and the like should all be avoided. Fake sugars such as aspertame and sucralose are ok in moderation.
Calorie laden drinks - Soda, fruit juice, milk, etc. It's ok to use milk for cooking, just don't use it as a beverage. Diet soda is ok as long as you still get all your water drinking done each day. Once in a great while I cheat and have Orange or Tomato juice with breakfast, but make sure you count the calories and mark on your checklist that you cheated on beverages for that day. Most items like grape or apple juice are full of sugar so stay away!
Bread - This has both flour and sugar in it, a deadly combination. Most bread is out! (If you look around carefully there are a few brands that are semi-ok, such as Sara Lee 100% Whole Wheat with Honey) Remember you can use thin slices of meat or lettuce as wraps for that ?sandwhich? type food. If you find yourself tempted to cheat on this one, reflect on the fact that bakers are stereotypically depicted as fat, that's not an accident! Fishermen are always shown as lean, wouldn't you rather be like that?
Corn - Corn isn't actually a vegetable, it's a grain and a really starchy one at that. Corn is ok in mixed vegetables, but don't eat it by itself.
Potatoes - Another very starchy item that shouldn't be on your menu. Both Russian and Irish cuisine involve lots of potatoes, and neither group is really envisioned as being a thin culture. Greeks and Japanese on the other hand don't and they generally are seen as being thin. Surprising? I think not.
Pasta - This isn't forbidden, however you need to be EXTREMELY careful here. First, make sure to only buy the newer healthy pastas (Barilla plus for example), and remember that pasta is very calorie dense, so watch your intake! Traditional white pasta is a serious no no.

I can't possibly live without insert bad product name here, what do I do? Well the first thing is to look around for healthier alternatives of the same product. For example they make a mayonnaise with Canola oil instead of Soybean oil now. If that doesn't work, look up how to make it yourself online, you may be able to substitute good ingredients for the bad ones. Using sweet potatoes in potato salad for example. Sometimes though, there just isn't any replacement or way around it, and you'll just have to decide whether you'd rather eat that product and be fat or skip it and be thin. As a last resort, remember that you only need 90% compliance to be successful, six meals a day seven days a week is 42 meals, so you could cheat at up to four of them, but that doesn't mean go hog wild, watch your portion sizes.