I attend a science fiction convention every year (
Norwescon) which has an emphasis on the business of writing and publication. One of the workshops I sometimes go to is panelled by publishers and agents, and addresses specifically how to pitch your book ideas to publishers and agents at a con.
The first think to keep in mind is that agents and publishers, especially at conventions with an emphasis on the business of writing and publication, are constantly getting solicited, so be respectful of their time, other commitments at the con and the fact that they probably have been too busy to get much in the way of sleep, food and relaxation.
Prepare an "elevator" pitch, ie something concise (and well practiced) enough to be given to an agent or publisher while you two share an elevator. No need to provide a full plot summary, just the hook given in two or three sentences. When you are in a position to give it -- in an elevator, in the hall way between workshops, etc. -- make eye contact, introduce yourself, offer to shake hands, then ask, "Can you spare sixty seconds to hear about the book I'm working on?" If the answer is No, provide a card (you
do have them, RIGHT? ) with your cell number and ask that, when she has the time, if she could give you a call and arrange a meeting of just a few minutes. If the answer is Yes, give your elevator pitch.
Have a copy of your manuscript with you. Nowadays, it is easy and inexpensive to put the first three chapters, a cover letter, a curriculum vitae if you have work experience relevant to your work and previous publications if any onto a CD and carry it around in case opportunity pops up. You should use standard CDs and not the mini; odd sizes are more likely to get lost or ignored. All documents should be in PDF, RTF or some other fairly universal format; if you use the latest version of Word, there is a chance that your audience will not have the programs to read it. Your name, contact info and the name of your work should be on the label, clearly visible on both the front and back. The same information should be on the CD itself, in case the disk gets separated from the case. After you give your pitch, offer a CD and thank the person for her time.
There is one very important disclaimer you should put in your cover letter, at the top of your chapters and as a separate document on your CD:
The enclosed copy of your work provides an example of your writing and does not constitute submission of a manuscript for publication. It is
very important that you make this clear. It is bad form to submit a manuscript to different publishers at the same time, and is likely to get you downpiled or outright ignored in the future. Be very upfront that you are looking for interest and possible feedback; if interest is there, you will submit the complete manuscript through normal channels. If you have already sold your first book and are only approaching agents, your disclaimer should state that the manuscript is already sold and to whom it has been sold.
Speaking of agents, another thing that the agents themselves have stated: they work for you, the author; you do not work for them. Be picky and look for someone who likes the kind of writing you do (a fan of your genre will be more assiduous in pitching your stuff) and -- very important here -- is able to have lunch with your editor. An agent in Los Angeles is probably not a good fit if your editor lives in Chicago, unless your agent travels regularly to Chicago anyway.