If you've decided to go to a conference chances are you'll be pitching your manuscript to an agent or editor. Packed full of great agents and editors, a conference is the perfect place to do some networking and actually get your work looked at. I don't recommend it for those who have just finished their book and just want to test the waters though. Approaching an agent or editor is a step you should take only when you think your manuscript is polished and in the best possible shape it can be. You want to look like a professional after all who takes pride in their writing and is willing to work hard to make it successful.
So now that both you and your manuscript are ready to go, how do you prepare? This can be one of the most nerve wracking experiences you've ever put yourself through if you approach it wrong. The point of the pitch is to tell an agent/editor about your book and then get their response. The second part is the really important part. Too many people make the mistake of talking through the whole pitch session. You need to hear what they have to say about and have the time to ask them questions. So what do you tell them about your book then? Easy, the same thing you'd tell them in a written pitch. If your query letter is good enough to send across the web or through post then it's good enough to tell them in person.
Memorize your query letter and plan on telling that to them. You don't want to sound like a recording but this will help you so you don't leave parts out. Just remember to be excited about your story and the chance to tell it to them, turn your nervousness into excitement. You love the story so let them see that. Also you should have a hook, one sentence that sums it up. Think of this sentence in terms of how a commenter would describe your book. It should be part of your query, the first part.
Once you've said your pitch they will most likely ask a few questions. Then it's your turn to ask questions. You should have already researched the agent and found out everything you can about them so you don't waste time asking personal questions about them or their agency. If you get the impression they aren't really impressed by your idea ask how you can improve it. Ask what editors are purchasing right now. Find out why they aren't interested!
On a positive note, be prepared for them to be interested. Have at least a sample of your manuscript, five to ten pages. Also bring a synopsis and a query letter with all your contact information on it. Most likely they won't ask for these things because the likely hood of losing it during packing and flying home are pretty high. They'll probably want your work to them. However, there are some agents who prefer the old fashioned way so it doesn't hurt to have hard copies of everything on hand just in case. I'd bring an entire hard copy of my manuscript just in case. You don't want them to be excited about it and then have nothing to give them.
If they ask to read your entire manuscript don't forget to ask them how they would like you to send it to them. Some will want you to e-mail it, others will want a hard copy sent through the mail. Believe me, when they ask to read it, you could be so excited you forget that important detail! Best of luck.
Here's a great link to important things to remember when pitching in person: http://blog.writersdigest.com/norules/2009/08/06/TheArtOfLivePitching3Rules.aspx