I'm not nearly as much an expert as many of the folks around here, but I've collected a few thoughts that I think are worth bearing in mind for beginners, so I thought I'd share a few of them.
1) Most people present their tricks just as stunts. I've seen many wonderful pieces of magic presented with no more care for scripting than to say "want to see something cool?" Try to actually present your magic. You can be serious or humorous and your presentation can be just a couple of key lines or it can be a fully-realized "script" with a full and rich narrative. That's for you to decide. But you must remember that all tricks need to be PRESENTED, and that your audience will never give them more importance than you do. I'd rather be seen to do something magical than merely "something cool." The former fosters a connection between myself and my audience; the latter sounds like something I just bought the secret to and learned how to do.
2) You must respect your audience. That doesn't mean you need to treat them with kid gloves all the time, but you should be mindful of their state of mind as you perform. Most importantly, you must never truly endanger your audience or their property. We've all seen those Russian Roulette routines go wrong and we've all seen or heard of magicians losing a borrowed item or returning the item in poor condition. It's okay to use these possibilities as a theatrical hook if you're careful to never actually make the audience truly uncomfortable, but it's never okay to actually risk harm to an audience member or their property.
3) Many will disagree with me on this, but I'm going to put it out there anyway. Secrets are important, and it's true you should not reveal the secret. But many magicians put too much stock in the method. An audience member who knows a method is only dangerous if the method is all you've got in your toolbox. If you actually PERFORM your magic, it doesn't matter if they know how it's done. We all know the secrets and we all still watch magic. Granted, we like it more than most people do, but I still maintain that the people who suffer from exposure are primarily only those who hide behind the method to mask their lack of skill at presentation. So follow the rules and don't expose secrets; but at the same time, don't avoid doing good magic just because you're afraid they might have seen it exposed on YouTube. Even if they have seen it (and they probably haven't), they'll still be entertained to see YOUR performance of it, right?
4) Magic that magicians like is not always the same as magic that audiences like. If you have a seventeen-phase card routine that symbolically traces the entire story of the Battle of Agincourt, I'd probably love watching it. And 99% of audiences wouldn't. That's not to say you shouldn't be ambitious with your presentations, but you should also keep things simple enough for people to follow them and just enjoy the magic. I get better reactions from salt shaker through the table than I do from my favorite five-phase oil and water routine.
5) You should study magic thoroughly. We're all tempted to buy that latest and greatest trick, and I'm no exception. Magic dealers stay in business largely because magicians like to buy toys, and there's nothing wrong with that. But you shouldn't ignore the classics. Even if you want to do a very modern cutting edge show, you should ground yourself in a world-class knowledge of your craft's history. Today's playwrights still study Shakespeare, and for very good reason. Further, regardless of what genre of magic you do, you ought to know something about the others. Even an illusionist benefits from a good knowledge of kid show magic, close up magic, mentalism, etc.
6) You should know more than just magic. Magic is a rare art that even if you kinda suck, you can get booked just because magic is such a strong form. But audiences aren't all that interested in the culture of magic (they have a curiosity, but that fades quickly when exposed to light). They're interested in people; specifically, audiences (especially audiences who can pay) are interested in well-educated people who can converse on a variety of subjects. The goal of magic should be to connect with your audience. Part of that connection is talking about interesting things. Be well-read. Plus, you never know where inspiration for a new routine may strike.
7) Appearance counts. If you perform in costume, this is obvious, but even if you don't, you need to pay attention to your appearance. Audiences have no interest in slobs (the exception being if your character is a slob and there's a clear distinction between your real self and your character). You should generally dress one level above the clientele. Make sure your clothes are neat, clean, well-tailored. Don't neglect your shoes. Make sure there's nothing in your teeth. Keep your hair in order. And remember, especially if you do close-up, that people will be staring at your hands. If you bite your nails, take action to stop it. Make sure your nails are neat and clean. Most important of all, make sure you don't smell. I'm not just talking about body odor, either--if you smoke, I'm not judging, but you better not smell like an ashtray when you perform.
8) Keep it professional. Fraternization with your audience may sometimes be tempting, and up to a point is even desirable as you DO want to connect with your audience. But keep in mind that you're not being paid to socialize. You're being paid to perform. Be friendly to everyone, and if you absolutely must socialize with someone beyond the confines of your performance, make sure you do it off the clock and out of sight, to avoid even the appearance of impropriety or un-professionalism. Never drink when you're performing. Even if you can hold your liquor well (and you may not as well as you think you can!), it's unprofessional to drink on the job. If you're performing in a setting where drinking is expected, you may think you can get away with it, and to some extent you can, but I'd recommend making an arrangement with the bartender to serve you only non-alcoholic drinks while you're working. You can be seen to be mingling with the guests as you're expected to, but without any risk of intoxication.
9) Despite its intrinsic theatrical value, magic is relatively low on the entertainment pecking order. Why? Because there are too many bad magicians out there. Don't be one of them. Make sure you can flawlessly present everything before you show it to anyone. And then show it to a couple of trusted "test audiences" before you show it to the public. That way, you've got any kinks worked out in private so the public only sees a polished, professional show. Do this even if you only perform socially or for friends and family.
10) Don't steal. It's fine to do someone else's trick if they've published it for others to use. But don't just copy them. Make it your own, at least in terms of presentation. Too many magicians try to be "clones" of their heroes. But think of it from the perspective of your audience: why should they hire a rip-off of [insert famous magician here] when they can go and actually see the real [famous magician]? Do your own show, in the way only you can.