I have my clients keep a journal as well. It does hold them accountable for what they eat. I always have trouble eating bad stuff if I know I would have to write it down and then show my trainer. Also people usually under estimate how much they eat and a food journal opens their eyes to the real number of calories they are taking in.
There are a few free apps that are available that work really well. I use MyFitness Pal. Beats having to right everything down.