Can go hand in hand with a fitness journal to keep track of your caloric intake and how much progress you make in the gym. Keep track of things like carbohydrates and proteins. The weight of each you ingest and how much fat it allows you, plan cheat days, plan healthy meals, keep track of vitamins and supplements that you take, when you buy them from, etc.
Clip low-fat recipes, take pictures of those protein shakes or that berry smoothie you just blended. include video footage of meal preparations, again, whatever you want your journal to be… It is yours.