Record everything you eat and drink. The most accurate food journal will most likely be the most helpful as well. Try to jot down or take note of everything that goes into your mouth. Include all meals, drinks, snacks and even nibbles of food you eat while you cook.
Be very specific, and break complicated foods down by ingredient. For example, instead of writing down "turkey sandwich," write out the quantity of bread, turkey and condiments as separate entries. Handle other mixed foods, like casseroles and smoothies, in a similar way. This will help you remember what is in foods or the total amount of calories.
Don't forget to record snacks or random odds and ends you eat, like a cookie offered at work.
Record all beverages. Don't forget to track your total water intake as well. Tracking how much water you drink will give you insight into whether or not you need to consume more water to help you stay hydrated.