Some snacks are healthy but very high in calories, and can sabotage your weight loss efforts if you overeat. For example, I really like the Planter's South Beach Diet Nut Mix, but one ounce is 170 calories and the can contains 10 ounces! It would be too easy to just open the can and start eating. I have bought those snack-sized baggies and as soon as I bring the can home from the grocery store, I divide it up into 10 little baggies.
I've done the same thing with other healthy snacks, such as Crunchmaster Multi-seed crackers. In fact, for me I need to do this with almost anything salty, otherwise I won't know when to stop!