I run the pitch sessions at my writer's conference, so I see all kinds of things. The first thing to remember is that agents are not ogre monsters. If you're nervous, they'll understand that. Most of them are nice people.
* Read the rules of the pitch sessions for your conferences and follow them. Some of what I mention below will be on the list.
* Make sure you have a completed manuscript for book-length fiction. We always get writers who show up with only three chapters done. We get the nasty grams from the agents afterwards.
* Don't pitch short stories. We always get one of these every year, too. We get complaints from the agents for this, too.
* Pitch to the appropriate agent. This year we had a lot of writers who showed up for pitch sessions to agents who didn't rep their genre. I guess the writers figured the agent would be so wowed that she would agree to rep the work. Instead, the writers were turned away without being able to pitch. We get complaints from the agents for this.
* Be on time. If you're bad with time, wear a watch and set the alarm. Write down the times and the names of the agents so that you remember who you're seeing and when. We had a lot of writers who showed up late or for the wrong time or couldn't remember who the agent was. One showed up half an hour late. His agent, concluding the guy was a no-show, had left for the day.
* Be friendly, pleasant, and polite. Smiling goes a long ways. If you've ever done customer service, put on your customer service face.
* The agent is not a celebrity! They're a small business owner looking for clients.
* Be assertive, but not aggressive. We've had a run of both, where one writer actually hid and another tried to force his way into the sessions. Neither is a good image.
* Know your story and your hook. If necessary, rehearse your pitch, though that depends on you. If I'd rehearsed for my two, I'd have screwed them up--I do better off the cuff.
* Don't waste time. We had one chatty writer who didn't get around to making his pitch session until time was up. He didn't do his pitch.
* Do bring your synopsis and first three chapters, but don't expect the agents to take the material. Both agents I pitched to asked me to email material to them.
* Do bring a small notepad and pen so that you can write down what the agent wants you to email, along with the email address. Ask them what they want in the subject line (they may have something referencing the conference).
* For any writers with special needs, please let the conference committee know. We had two at the last conference (one wheelchair, one with a walker), and we could have scheduled their pitch sessions together so they only had to make one trip downstairs. By the way, just to show you what agents are like, the woman with the walker stayed in the room to wait for her second pitch session and sat at the empty table next to us. When it was time, her agent hopped right over to the table so she wouldn't have to get up and had the pitch session right there.
And if there are any opportunities to attend an agent lunch or breakfast, take it--even if the available agent doesn't rep your genre. It's a chance to ask any questions you may have.