1. Set a goal.What are you hoping to achieve by keeping a food diary? This will help determine what kind of format you use and what sort of data to track. If you want to gain or lose weight, tracking the number of calories you consume will be important. If you want to make sure you are getting the right balance of nutrients (protein, fiber, and so on), you will need to have that data broken out for each meal or snack you consume. If you are trying to determine if you have a sensitivity to certain foods or ingredients, you may want advice from a physician or nutritionist.
2. Choose a system.
How will you record what you eat? Your food diary can come in the form of a paper notebook, an online tracker, or a mobile app. If you don't have a smartphone, then a combination of pen and paper (while you're away from home) with an online tracker might be a good idea. With apps and online diaries, you'll save yourself some math since calories and nutrients will be totaled for you.
There are many, many options out there and you might need to try a few until you find one that works well for you. About.com's Calorie Count is just one online tool that will help you keep a food diary and tap into other members, as well as registered dietitians, for advice and support.
Mobile apps can be a great choice since you usually have your phone with you when you're not at home. Many also have barcode scanners that let you add new foods (and all their nutritional information) very easily. LoseIt! (pictured) and MyFitnessPal are two popular options.
Make sure the system you choose works for your lifestyle. And make sure it can capture the data you need to collect. Aside from nutritional information, you might want to keep track of how hungry you are when you eat, or what emotions you're feeling before or after a meal.
3. Get started.Don't wait until next Monday, the first of the month, or after the holidays. Start now! The first several days may go slowly, because you'll need to look up data and measure your portions. But you'll get better at estimating portion sizes with practice. And if you use an electronic food diary, you can add your favorite foods to its database and even create custom entries that you reuse often (like "my breakfast," consisting of 1 cup of nonfat, plain yogurt, 1/2 cup of blueberries, and 1/4 cup of your favorite brand of granola). Don't forget to record beverages, because the calories and sugar in drinks can add up in a surprising way.
4. Stick with it.Some days are easier than others, but keep going. If you're in a rush, take a quick picture of your plate, then refer to it later when you add foods to your diary. You could also leave yourself a voice memo or jot down a few quick notes. When you eat out, look for nutritional information online—ahead of time, if possible, so you know what to order. But afterwards works too. You can log in the details of the meal you ate, and see if another choice would be better next time.
5. Look for patterns and trends.With a mobile app or online tracker, you'll probably have charts and graphs that show you how your daily calories and nutrients stack up against your goals. You can get the same information with a pen-and-paper diary (you'll just have to look up calorie data and add it up yourself).
Along with showing you how you're doing against any goals or guidelines you have, your diary can also help you identify trouble spots: Are you skipping breakfast often? Snacking a lot in the evenings? Hitting the drive-through a bit too often? See a list of questions to answer while you review your diary entries.