My favorite food diary tool is My Fitness Pal. Using this handy app you can easy search for your food, indicate how much you ate and hey-presto it tells you it's nutritional value and subtracts it from your daily calorie allowance.The data is fascinating and by logging your food in a daily diary (hard copy or electronic) you can monitor the volume of food you're eating as well as learn a few surprises about the what you've been eating - foods you think are "healthy" can also be calorie-loaded.
After a while you can get diary-fatigue and possibly not log in each day because you know you've been eating too much.
An alternative way to use these same tools is to plan your daily menu rather than log it retrospectively. Each evening or over breakfast you can plan out your menu for the day. The benefits of this approach to My Fitness Pal and similar database tools are:
My personal opinion is that following a set menu is a large part of the success that organised diet plans have. With valuable tools like My Fitness Pal and other databases, you don't need to buy Menu Plans - you can do them yourself.