Don't buy any snacks, you can't eat what you don't have! Eat 4 smaller meals a day instead of 2 or 3 larger ones and if you do have to pick in between meals try eating raw veg, carrots, peppers, celery etc rather than fruit as they contain less sugar and are better for you anyway.
Dont buy snacks/junk food when snacking this sounds hard to do but the trick is not to go shopping on an empty stomach have breakfast/lunch/tea just before shopping and you will find you have no interest in the snacks as you feel full!
Sounds stupid but it really does work :)