Can't remember if this has been suggested, but chewing gum/mints might be an idea - you'll have something in your mouth that way, and the minty taste will put you off eating for a while when you've finished it (though for max. effect, if you prefer mints, go for the sort that are v. minty and take a long time to suck, like extra-strong ones.) Every time you feel like eating something, grab a mint instead.
Alternatively, if you actually want something to eat try not having any junk food in the house and buying a big bag of crunchy apples - that way if you succumb to a food craving at least it'll be fruit, which is healthy and not fattening. Plus crunchy apples are lovely and take a bit of time to eat, so you'll be less likely to go and eat something else.