Offer to chair a session - how should I decide whether to accept?

Yes, you should.

Whether this is common depends very much on the conference. The last time I was at a conference, I was invited out of the blue by someone I didn't know to chair a session, in a very short and very informal e-mail as if it was coming from a long time friend in a hurry. This was less than a year after I obtained my PhD. A PhD student in our research group did the same.

All it means, really, is that you sit on a chair in front of the room, make sure the presentations are on the presentation computer, announce the speakers, etc. You will get detailed instructions. Unless you mess up (and why would you?), you really have nothing to lose.

The typical duties of a session chair are:

• Before the session, make sure all of the presenters are present and check A/V to minimize problems during the session.
• Convene the session at its start, getting the audience to sit down and be quiet.
• Introduce each presenter and their talk.
• Keep the session on time, warning a presenters when they approach the end of their allotted time and cutting them off if necessary. This is especially important for big conferences with many parallel sessions, where people may be switching rooms in mid-session. You should typically ensure that about 5 minutes is reserved for Q&A, but the time may be shorter with very short talks (e.g., 3 minutes of a 15 minute talk).
• Moderate the question and answer period.
• Formulate a couple of interesting and respectful questions of your own for each talk, in case the audience doesn't have any questions. This is sometimes the hardest part of moderation.

These duties aren't too hard, and it's not too unusual for senior graduate students, postdocs or young faculty to be asked to chair. It's a nice little low-grade visibility and networking experience, and can be a first (small) step towards becoming involved with other aspects of conference organizing.

The only reason that I might recommend against accepting is if the session is early in the conference and you haven't been to conferences and seen how sessions are typically run before. This is something where I would recommend making sure you've watched others before you do it yourself---though you will see many bad examples as well as good.