First off, congrats!
Second, from your language I would guess you are in a large American city. This tells me a little about you, and because of that I would like to recommend something. Make em laugh. A good salesman, will seem to be your friend (making you trustworthy), and a salesman second. In reality it's the other way around. When you can make someone laugh - but not by making fun of someone else, you can open them up, and get them to relax a bit. If they relax, they are more likely to follow what you say or tell them to do. Just make sure you laugh too. I know it seems strange, but even if you have heard the same joke, 1500 times, laugh. Don't giggle, don't do what sounds like a fake laugh, in fact practice a couple laughs. Laughing also relaxes your "mark".
Last, wear a suit. You might only have a cheap suit right now... if you can afford a more expensive one, then do so. However if you can't afford it, be aware that great suits can be found at GoodWills, Salvation Army, or other thrift/second-hand stores, just get them tailored to you. And remember to shop around. A Wool suit is the best, a polyester blend might look nice, but it's considered cheaper - it's still nice to wear one if that's all you can afford however. You should dryclean the suit at least every 7 days of wearing it, but 1-2 days is better.