First, things to remember:

• You are not a person. You are, at best, a fantasy given shape. He doesn't care about your problems (unless he can fix them and feel heroic) and he doesn't care about your hangups (they will be looked upon as challenges or shortcomings) and he doesn't care about your history or your education or your friends or your opinions, unless they make a cute and forgettable anecdote.
• It won't last forever. As a mere novelty, his affections will endure only until he's seen all your superficial tricks of charm. Don't be confused by I love you's or talk of some distant future. He'll get bored, and you should be ready.
• Having a sugar daddy will be of no use to you. If you're not emotionally prepared, it will, in fact, be to your detriment. However, the singular charm of a sugar daddy is the material goods he will ply you with. Get over your ethical delusions and take him for all he's worth. He's not going to respect you for turning him down, and if you're not asking, merely receiving politely, it can't really be said that any loss to him wasn't his own choice.
• You can do better. So maybe at first he strikes you as sophisticated and brilliant, and you're drawn to his lifestyle. You'll reach that age, too. You'll be all those things yourself, and able to play sugar mama to smooth-skinned young boys, should you choose to do so. You're young. You could be having better sex, less pretentious conversation, beer and pizza, and wild adventures of the sort that he'll be uninterested in sharing with you. When it's over, you'll move on, and do it far more quickly than he.

Supposing you know what you're doing and want it to continue as long as possible, these are the things you should be doing:

• Maintain your appearance. The stringency of this precept varies from person to person. Don't pierce anything or get tattooed unless he's paying for it and finds it sexy. Don't make big changes to your hairstyle or makeup unless he's trendy and prone to do the same. No matter who he is, maintain your figure. No matter what your body looked like when he met you, you can rest assured that that's what he wants. But if he tries to talk you into to surgery you don't want, lose him.
• Say nothing of substance. The difficulty of this is proportionate to his intelligence. If he's a moron, he'll be charmed by giggles, cliche mischievousness, and body language. If he's a little smarter, tell him things he knows, agree with his opinions and expand in a non-threatening way. Never do anything to indicate your intelligence. And, either way, listen attentively in a way that doesn't make him suspect he's dominating the hell out of the conversation and boring you half to death. Sugar daddies often think you want the benefit of their knowledge and experience, when generally all they're good for is the benefits of their credit cards.
• Impress his friends. If he respects them, be demure. Treat them as you would him. If he thinks he's better than them, be witty, even gregarious. Just don't screw up and take that tone with him. If they're potential romantic interests, charm the hell out of them, but in a way designed to charm him even further. Throw everything you've got into the conversation and flirt subtly. If the competition tries to call your bluff and out-vamp you, play innocent and naive, if the situation permits it. Otherwise, act bored. Any hint of jealousy will work to your immediate disadvantage.
• Don't ask for things. The words, 'Will you buy me that?' should never cross your lips. Hint, but be subtle, unless what you're aiming to get your hands on is truly worth letting him make you feel dirty over. Chances are it's not. Be patient.
• Move on graciously. At the inevitable end, it should appear to everyone but you that you've just ended a fascinating and contented relationship with someone you respect. Not that you've just sucessfully conned some idiot out of.. whatever. If you begin to hear it from other people, it might get to you, and the idea is no harm, no foul.

While I can't really recommend this (you're brilliant enough to be your own sugar daddy), it's better to go into it aware of what the game entails.