I attend 2-3 conferences a year. Unfortunately, most of the professors I would like to work with don't go to conferences (boooo!) But I love the conference world, and I have learned SO MUCH from going to them, and met so many people I really enjoy and appreciate knowing. Here's what I do:
1. I go through the program ahead of time, looking for names of people I specifically want to meet or speak with. I also choose the sessions I just "have" to attend, and take a close look at session times where there are more than one I would like to attend, or none I would like to attend. In the case of the former, my choice is usually dictated by who's on the panel; in the case of the latter, I check to see who I want to meet and speak with and try to determine whether this would be a good time to go for a cup of coffee.
2. Staying at the conference hotel is definitely a must. One year I did not, and I didn't really get the chance to speak at length with anyone. Also, after hours activities tend to happen in various conference hotel rooms, and if you aren't there this can be a disadvantage for you.
3. Go to at least one panel where you KNOW you know a lot about the topics being spoken on. In that panel, participate in the Q&A. After it, stick around to speak with the panelists and the moderator. This nearly always swings you the chance to meet other people and - especially in panels before meals - can usually get you an invitation to join the group for lunch or dinner. Which is where the informal but very important collegiality moments take place.
4.Don't be afraid to introduce yourself to people you want to speak with, but make sure you have something to say other than "I love your book". I made that mistake once and felt SO dumb. A good back up is to ask what ongoing projects the other person is currently working on. This can usually lead to a decent discussion.
5. Unless the conversation seems to be going really well, don't expect one to last more than about 8-10 minutes. Don't be insulted or worried you offended someone if s/he looks away while talking to you or abruptly excuses him or herself. Remember, they are also seeing one another for the one time this year (or in the past few years, or whatever) and they have their own agendas and people they want/need to talk to. Also, academics are socially awkward in many cases. If the discussion is going badly, just quietly think "it's not me, it's them". As long as you are polite and friendly, you are doing fine.
6. Get email addresses, Facebook friend invites, suggestions for where to present papers you are working on, research leads - talk to anyone you want to talk to, pick their brains, they love it. I had the best experience this past conference in a session that was fascinating, but way outside of my scope of study. I went to one of the speakers afterwards and said "So, as a complete neophyte in this area, I thought your paper was extremely interesting, and I'd like to know more. Where would you point me if I wanted to begin reading into this subject a little further?" Turned into a lovely chat for half an hour over coffee and over a page of suggested readings for me, and for her, the opportunity to impress someone listening in (and currently hiring in her field) enough with her off-the-cuff knowledge of resources to warrant a "why don't you send us your CV?" A great, win-win moment.
7. Toe the line between being knowledgeable in your field, and interested in but deferential towards other scholars in other fields of study. This is hard, because you have such limited time and you're so focused and obsessed on getting as much as you can about your subject field from the conference. But trying to be interested in subjects you usually don't work in does pay off in terms of those scholars thinking highly of you for your outgoing and friendly demeanor, and they in turn can introduce you to people you otherwise might not get a chance to meet but might want to. Accept any and all offers of introductions.
8. Don't be a wallflower. There's no point in going to a conference if you are not going to meet people. Talk to anyone and everyone you can talk to. If you are socially awkward (and I am, believe me) talk yourself into a conversation by making a comment about the food, the coffee, the buffet, the book sale, the panel you just attended - someone will pick up on what you say and agree, and then you can introduce yourself and make small talk ("Hi, I'm Medievalmaniac. And you are? Oh, Such and such university! What subject are you in? Are you giving a paper?" etc. etc. etc. The important thing is to meet other people and to get your name and face out there.
9. Panel etiquette - if you agree with or enjoy the papers, or have actually questions to clarify or to deepen what's been said, comment effusively and openly during the Q&A. But if you disagree or know what the panelist has said is incorrect, save this for a personal conversation after the panel has concluded and once the room is cleared. There are plenty of people who go to panels and openly negate the panelist's work, but they are a.) tenured (hopefully) and b. established enough to not care what anyone else thinks. Arguing with a point or two is one thing, the panelists enjoy debating; but open criticism doesn't make you look smarter or more knowledgeable, it makes you look like an ass. I once sat in a session in which one of the top scholars in the field gave a paper, and then a second year graduate student on the panel gave a paper that specifically denigrated the top scholar's work and criticized his approach as flagrantly wrong. The comments made after the session and over coffee about the graduate student were not very flattering, and ran along the lines of 1. Who does he think he is? 2. Why didn't his advisor warn him not to do that? and 3. Didn't he know who was on the panel with him when he made the decision to say that? In other words...bad form.
And that's essentially what is required - good form. Just be polite, be open, and enjoy yourself, and it will be a fantastic experience. I am socially awkward, but I LOVE conferences. There's nothing like the ideas high you get during and after a conference to spur you on to working on your own research - and once you know a couple of people, it definitely gets to be more fun!!