On pitching in general
I HATE pitching, hate it with a passion. That said, I landed my present agent, and several other representation offers, after doing pitches--when 22 queries on the same novel didn't even get me requests for partials.
(I should mention that I had to attend a previous conference and ptich unsuccessfully just to get a feel for the process. It's pretty weird.)
I think that face-to-face pitching works better when you have something that is complex to explain or contains an unlikely juxtaposition of material. Agents reading queries don't want to see anything too complicated, and they immediately tend to steroetype.
Pitching a book allows you to throw in the "BUTs" and answer questions. By throwing in the BUTs, I mean it lets you explain that this is a literary novel about a young man dying of AIDs BUT it's also an upbeat comedy, or that this is a book where the plot sounds like Tom Clancy BUT it is written with the sensibility of Dennis Lehane (or Kurt Vonnegut, or Ann Beattie).
Pitching allows you to say that this is yet another book of historical non-fiction about the Civil War BUT you've written it like early Tom Wolfe. (There is less of a problem with this in non-fiction, of course, since you get to send an entire book proposal. Us fictioneers really get the shaft in the query process.)
Some novels with interweaving storylines are almost impossible to synopsize, even though they may read easily enough. ('Meanwhile back at the ranch' doesn't work well in synopses.) It's easier to squeeze this in to pitches.
But above all, pitches allow the agent to ask you questions, and lets you address their specific concerns. So the number one rule is: Don't talk too much. If this is a ten-minute pitch, plan on only using one or two minutes with your actual pitch, and then give the agent time to talk.
Ten minutes is much longer than you think. Don't be afraid to take time chatting.
And above all, try to have fun.
Good luck!