Kepp your back straight, speak briskly and loudly, essentially assert yourself. Be brief, as concise as possible, concrete above all else, do not expect any sympathy whatsoever and don't bring up emotional matters, because you might as well be talking to a brick wall.
Be polite and respectful, never make the mistake of expecting understanding, prepare yourself for a lot of criticism, do not complain about anything, if your life or your beliefs are dear to you keep your idiosyncrasies to yourself. Do not ever talk in a vague, long winded, detailed manner, this will make them discount you as a blabbering fool, because if you have no immediately useful things to present they will just ignore you.
Or you can do what I do. Be a stubborn ass and get your jollies out of winding them up.