When a Domme talks to you.
Obviously be polite. We all like politeness. Dommes usually insist on it. However, don't grovel, since we don't like that. Most Dommes don't like it. Remember, that she is talking to you because she finds you interesting - the fact that you're a submissive is taken for granted, so you need to make sure that you think in those terms as well. Do not immediately launch into a sales pitch about how you're the best sub she'll ever meet. She won't believe you and she'll lose interest within seconds. Of course, if she asks what you like doing as a submissive, tell her! DO NOT SAY 'ANYTHING YOU WANT MISTRESS'. This immediately puts you into the category of sad desperate loser. Tell her what you do like doing, and what you don't like doing. Be clear about this, since she's going to find out at some point, and you might as well be up front about it - it'll save your time and hers.