Anna is right on the money with most of her suggestions, but I want to add a few more. First, I would make sure that you dress conservatively- that means you need to wear clean clothes which are appropriate for a business environment, such as a skirt or dress slacks, and a matching top that is NOT revealing or risque in appearance. Keep your clothes simple- no dangling jewelry, no visible body piercings ( now is NOT the time to show off your nose ring, or your tongue stud, or that cool tattoo you may have on your rear end) and keep your makeup to an absolute minimum. Your hair needs to be clean and neatly styled, in a conservative manner ( NO Mohawks, no spiky hairdo's or weird cuts or styles, and limit your use of hairspray and mousse) and you also need to make sure that you bathe and brush your teeth before you leave for your interview. NO PERFUME or cologne- many people are allergic to strong scents, and adults are frequently turned off by people who use too much perfume. Use some deodorant, however- having BO is a sure way NOT to get hired.
Other hints on the subject of appearance: make sure that you go to bed early on Thursday night and get a good night's sleep- no one EVER can present herself well to a potential employer when she is tired. Eat breakfast ( or whichever meal is appropriate to the time of day) BEFORE you leave the house, because you need the energy and because this will help calm your nerves. Lastly, plan ahead and lay your interview clothes and accessories out the night before, so you are not scrambling around frantically trying to find things at the last minute. Having to rush because you couldn't find something like your shoes can really stress you out and make it impossible to present yourself well in the interview.
Now, about the interview itself: First of all, I can't stress enough the importance of being ON TIME, or even a few minutes early. Showing up even a few minutes LATE does NOT say good things about you- it will tell your interviewer that you aren't responsible and that you don't care enough about your job to be on time- and that is hardly the message you want to send to someone who is thinking of having you look after his or her children. You need greet your interviewer(s) in a pleasant, professional manner, and shake hands with them. If the children you are being interviewed about are in the room with their parents, you should ask to be introduced to them, and use the children's names. Make eye contact- this means that when you are asked questions, you need to LOOK at the person you are speaking to, and not look around the room or at the wall, or over their shoulder. Lastly, you need to have a list of your OWN questions prepared, and ask in a pleasant manner about your duties and what other responsibilities you will have. Watch your language- speak in complete sentences, and DON'T interrupt. It also goes without saying that this is NOT the time to speak in ghetto talk, or to use a lot of slang terms or profanity. Make sure you use correct grammar as well- that shows the parents that you are an educated person who is serious about school.
Finally, LEAVE YOUR CELL PHONE AT HOME- and this goes double for your ipod. There is nothing which will turn a potential employer off faster than you showing total disrespect by having your phone go off in the middle of an interview, or by showing up wearing headphones and spaced out on music. Adults do NOT like either of these situations.
Good luck, and I hope this helps you.