Michael Pollan references the 5-ingredient rule of thumb. He also suggests using "Don't buy anything with ingredients your grandmother wouldn't recognize."
In other words, if the ingredients start sounding more like a science experiment, don't buy it.
I live in a developing country, where processed foods are rare and extremely expensive. I am forced to buy most food fresh. It's been a great awareness exercise. Most of our grocery money is spent on fresh fruits, vegetables, meats, poultry, and dairy. These are bought from local markets. I only buy from the supermarket the other stuff: peanut butter, pasta noodles, grains, canola oil.. not much else.
Our pantry mainly consists of spices, different vinegars and oils, rice, semolina and other grains, and legumes. Our refrigerator is fresh fruit, vegetables, milk, eggs, yogurt, and cheese. We make most of our meals from these ingredients.
If you are willing to put the time in to cook (which I LOVE so it's easy for me to make time for cooking!) you can easily build a diet around fresh, unprocessed foods. We'll make a quiche for breakfast and eat it for a few days, or muffins made from bananas and soy flour for protein. Dinner may be home made pasta sauce (made from fresh tomatoes- you can just make a big batch every now and then and freeze it) with eggplant or pasta, a casserole, stir fry, baked fish with fresh herbs, or an Indian dish (spiced lentils, channa masala, etc), with sauteed vegetables, cauliflower soup, or fruit salad for a side.
I share all this because it has really changed my perspective living in a country where I can't get packaged foods. Even though I shopped at Whole Foods in the US, I was still getting a lot of my food from boxes and packages. I didn't realize that until I moved overseas, when there were far few boxes and packages to buy