Officially, it has been determined that while getting more rest and consuming healthier foods is of course better for you, there is no direct link to the old wives' tales and acne. It is generally caused by genetics (as in you were screwed no matter what you did) and then a lack of care. If you wear makeup, sweat all day, and then don't wash your face, hydrate your face, and treat little breakouts as they come up, you will inevitably end up with a form of acne.
Instead of stressing about that chocolate bar you indulged in or those onion rings you got with your friends this weekend, remember the only thing you can do about acne is take care of your skin. Use sun screen, wash your face morning and evening, always wash makeup off before bed, use an appropriate moisturizer for your skin type, and use simple things like salicylic acid treatments on pimples to head them off before they get too bad.
Some people unfortunately can't do anything about it and are going to end up with bad boils and cystic acne. This is bacteria on your skin. Bacteria in your body means you go to the doctors. This is no different. See a dermatologist. They'll prescribe topical ointments and sometimes oral medication to treat it. It is NOT worth suffering the embarrassment, discomfort, and all the other costs and negatives that accompany acne.
** And remember: IT IS NOT YOUR FAULT! **