I had a diary when I was in elementary school. I wasn't very consistent in my usuage of my diary, maybe writing in it once a month. I wasn't a boy crazy kid who gushed to her diary about all the boys I had crushes on. Instead, I would occasionally write about what I did that day. I didn't really see the point.
Food diaries, on the other hand, have a valuable role in making wise food decisions. A 2008 research study praised the benefits of keeping a regular food diary, especially when trying to lose weight or follow a special diet. People who kept regular food diaries lost TWICE as much weight as people who did NOT keep a food diary following the same diet. People who were following specific nutrition recommendations, such as those for diabetes or heart disease, were significantly more compliant if they kept a food diary than if they did not.
I encourage my clients to keep a food diary when they are starting off on a nutrition goal to help increase their awareness of what they are taking in, and how it adds up in the day. I suggest trying a food diary for a week or two until you are able to establish a pattern of intake that meets your needs and goals. After that, I would encourage you to keep a food diary a couple days a week, but don't feel stressed to do it EVERYDAY.
When keeping a food diary it is important to think about every. single. thing. that goes into your mouth. So that includes drinks, condiments, little pieces of candy you grab off the receptionist's desk, etc. It is also important to pay attention to how a food is prepared (baked, fried, grilled, etc) and a specific portion size. You may even wish to measure your food for a few days to get a good visual picture of what your portions look like.
Here are a couple of websites which have good food diary programs:
Have you ever kept a food diary? How'd it work for you?