Actually, before the meeting.
Make a list of people you want to meet. Or that you know you ought to go meet if you weren't feeling so damn shy. Print out an abstract or three to read during transit. Practice saying "Hello, Dr. Gen-X, I read your work X and thought YZ about it. I'd really like to talk to you about it". YZ works best if it is a single phrase, not too flowery and scientific in content.
If you are an Asst Prof coming up for tenure, think about who is writing you letters. Go talk to them. If you are a postdoc, don't just talk to the people you know have or suspect will have jobs. Go to talk to people you might like to work with. Don't necessarily ask for a job. Say hello. Mention what you're doing. Ask them about their work. If you are a grad student, go to people in whose labs you might like to work. Know the application schedule and do this before you apply. Ask if they are taking new students. Show that you have read some papers, and indicate why you want to work there.
If all seems madness (for example during poster session), ask for time with a cup of coffee. Or even say, is there a better time we could talk? Do not be disappointed if they say no. And do not underestimate the value of a 2 min convo during the poster session.
The important point: do your homework before you go. Next most important thing: do the follow-up. Send an email.