For those I've helped rid themselves of passive-agressiveness, one rule seemed to help them all equally.
If you haven't asserted your desire or told the other person what bothered you about a situation in
one day, you have to forgive and forget. You have one day to let the other person know how you feel, why you feel that way, and why they should care. If you fail to let them know, it's YOUR fault. They won't know unless you tell them. You have ONE day to let your feelings be known. After that, you've dropped the ball - not them. If they eat your food and you say nothing, they have no way of knowing that it bothers you. If you haven't told them in a day, they're going to continue doing it because they're oblivious (or pretending to be oblivious) to the concern it causes you. Unless you're firm and vocal about your needs and desires, people will make many assumptions about what you're okay with. There are things my friends may do around other people, but as part of a mature relationship my friends and I have established what OUR rules are. Everyone has different rules, and unless you tell them -- they'll never know. The "one day" idea just strengthens the rule; you have a deadline. You have ONE DAY to tell them. Do it now!
One caveat; if you continue to express (a) why it bothers you, (b) why you feel the way you do and (c) why they should care, and they continue to disregard your advising, then you would be better served mentally to remove them from the list of people you care about.