My first tip is to cut out everything that's processed (includes "additives"). Your paying alot more for that garbage than you think. For instance, if you want spagetti, buy a can of crushed tomatoes, fresh mushrooms and add your own meat and italian seasonings. Same goes for mac & cheese, and just about everything else you can buy premixed. Its amazing how much more flavorful, healthy (its unholy how much salt and bad stuff is in those things) and cheap it is. And the time spent preparing is just as quick. I love hitting the site www.allrecipes.com for nutritious alternatives to common meals. If you can think of it, they'll have about 50 recipes for how to make it, and each one is rated so your meal wont be a flop.
Next, buy alot of your fruits & veggies frozen so they wont go to waste. These are also items you can buy in bulk - broccoli, green beans, strawberries, etc. Sam's also carries some things that WM doesnt. Also, most fruits and veggis can be bought (or grown) fresh and then frozen - peaches, strawberries, blueberries, pears, etc. Some veggies will need to be blanched first, but easy and quich enough to do. I cant overstress the backyard garden tho, and kids love helping with this part. We've offset a significant amount of our summer veggie cost with green beans (we currently have about 12lbs in our freezer), squash, zuccini, cucumbers, eggplant, roma tomatoes (again, good to freeze - I have enough for sauces to last all winter), etc. And just this year we planted several fruit trees, grapes and a blueberry bush. My girls also love taking our surplus to school and the Farmer's Market (which further extends the budget, it only costs $10 to set up a "stand").
Lastly, fruits and veggies are cheaper than meat, and healthier - I've made it a rule in my home to put fruit & veg as the main course and meat as a side. Typically, a lb of meat will feed my family of 4 with leftovers. Stirfrys, stroganoffs & casseroles are good all-in-1 meals, and most of them can be frozen prior to cooking, also eliminating the need for boxed frozen meals. Just double your recipe, split, and put your extra in a covered Pyrex or Corningware dish.
I hope this helps. If you'd like some more ideas, I'd love to share. I've already eliminated MSG, corn syrup, fried-vegetable oils and artificial additives to my family's diet. Everyone's in trim shape, and we havent had an illness in years. :}