Well that's typically largely out of your control. However, there are a few things that I find frequently inspire people to AFK, which you can probably avoid, or help mitigate:
Banning a declared champ, stealing a
pick, or trying to micromamage things
like summoner spells or matchups.
People get defensive about these
things, and the disadvantage in their
matchup probably isn't as big as the
handicap given to them by tilting them
before the game even starts. Try to run
interference for things like that when
you see it, as well. It may not save you,
but it might inspire good habits in
others. At the very least, it helps
de-escalate and already bad situation.