Healthy low calorie snacks is an easy one. Fruits and vegetables of course, then it's a matter of how much you eat. Here are some guide lines:
broccoli
cauliflower
celery
cucumbers
radishes
These are probably the least amount of calories followed by carrots. But we don't have to stay with vegetables. There are fat and sugar free:>
ice pops
frozen grapes
sugar free jello
Laughing Cow light cheese
air popped popcorn
nuts are great. They are filling, not too many though!
And then there is V-8 Juice, low calories watch the salt content. Small box of raisins, about 130 calories. A little bit of peanut butter used creatively or by it's self will serve you well. Babybel light cheese. Last but not least a good quality beef jerky.
I think pre-packaging your snacks is the best thing one can do to control your appetite.