I've edited this entry to reflect the new list of helpful websites to the right.
I use the following rules, it makes shopping easy: Just try changing one product at a time. Eventually, when you run out of the old stuff and replace with different, healthy versions, you will begin to feel better.
No white flour (disguised as enriched flour)
No white sugar or sugar substitutes (look for fruit juice sweetened, or organic sugar)
No artificial flavors
No artificial colors
No partially-hydrogenated oil (look for expeller pressed oils)
No corn syrup or high fructose cornsyrup (I stand corrected, our bodies glycemic index skyrockets)
It may take an extra few minutes to read the labels, but once you find the healthy version of the foods you like, put them on your grocery list.