There are a few rules which I stick very strongly by.

1. No one touches my food before I do. I am perfectly happy with sharing and you can help yourself to as much of my food as you wish (within reasonable bounds obviously)... As long as I take the first bite. All of my closest friends know this (and it's become a bit of an affectionate joke) but no one - NO ONE - touches my food before I do.

2. Knock before entering. This is a basic practice of etiquette and it bugs the FLIPPING HELL out of me every time someone enters without knocking. You knock and then you enter. You don't need to wait for me to tell you to come in - if the door is unlocked go ahead and turn the doorknob. Obviously if you hear me scream you shouldn't proceed with entering. But always. Knock. Before. Entering. It is one of the most elementary rules of politeness.

3. Say please. I'm not insanely picky about this: If you say "Do you mind handing me that?" or "May I have this" that's fine, I think probably because the "mind" and "may" provide sufficient politness. But don't go "Can I have that?" or "Get me a drink". You say please or I'm not moving. Same rule applies to thank you.