Do you keep a food and exercise diary diary?

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I've recently started writing down what i eat, drink and how much i exercise. I have been finding that I'm less likely to cheat if i have to write it down. Do you do this? How long have you done it and do you find it effective? On the computer or on paper? I've been doing both computer and paper - computer through wellness360 as its a site that apparently pays its users to use it.