I had the same problem with habitual snacking. Basically I had to brainwash myself into new habits, which meant forcing myself to stop before eating anything and have a bit of a think about it. If I want to eat I ask myself why. Am I bored? Do I have a craving? Or am I genuinely hungry? I try drinking water to make sure it's not just thirst. And I keep fruit handy for if I'm really hungry. Now I think I get sick of my constant psychoanalysis of myself and I just get used to a small level of hunger until my next meal-time rather than go through all the self-interrogation!
My problem now isn't habitual snacking, but strong cravings for certain foods. Tonight I had a real craving for potato chips. I knew I couldn't stick to just a couple, and I was already close to my calorie limit, so I waited it out until the craving passed. I've found if I distract myself with other things (reading a book, watching a movie, or even eating a piece of fruit) the craving often passes or at least becomes easier to deal with.
My other way I'm trying to deal with cravings is seeing if I can satisfy that craving with something near enough but healthier. If I'm craving chips it could be the crunchy texture I want, so I have a Ryvita with some hommus instead. If I want lollies, maybe it's the sweetness I'm after, so I'll try fruit instead. And if it's chocolate I'm desperate for sometimes a low-cal hot chocolate or a small piece of dark chocolate can satisfy the cravings with a fraction of the calories or fat. And sometimes a nice cup of tea is an effective substitute for a glass of wine (sometimes, not often though!)
I think healthy snacking can be good if you really savour the taste. Don't just pop it in your mouth and chew. Make sure you thoroughly enjoy every morsel of your snack. It will take you longer to eat so you'll end up consuming less, and you'll probably get more enjoyment out of it. And this will also give your head a chance to catch up with your snacking. If you can't stop yourself from snacking before it happens, at least you'll be aware of what you're doing while you're doing it. It will become more of a Big Deal instead of something you do on auto-pilot and it could help you make conscious decisions about what to snack on and when to do it in the future.