Many better people than I have written about why diets don't work, but this little piece of research kind of sums it all up. Will power requires a pretty high blood glucose level. Excellent, all I need to do to have the will power to avoid eating junk food is eat sugar. The research was done on other kinds of self control, but it seems like an obvious extension.
I did also find it strangely comforting that I am not the only one who sets off to do some serious work with biscuits or chocolate.