Getting away with an info-dump

Info-dumps are generally a bad idea.
The info-dump, as writers reading this undoubtedly know, is the part of the story that exists purely to provide information, and lots of it. A throwaway line is legitimate (“I haven’t been back to that school since I was expelled for selling pot to the archbishop.”); a long lecture (“I had very little money left over after Mom paid the school fees, so I started supplementing my income by selling pot. I set up a grow room in the house …”) not so much.
If you feel it’s absolutely essential to include the information, the questions to ask are:
•Is it interesting? The time-honored rule is that just because we find our characters fascinating, that doesn’t mean the audience will.
•Is it relevant to the story? I remember one of the Riverworld books in which Philip José Farmer reveals that one protagonist lost his virginity in the back row of the church while it was empty. Relevance to anything that happened to him in the book? Nil. Interest? Also nil.
•If it’s revealed in conversation, is there a reason? Telling people stuff they already know is the classic of bad info-dumping.
Alternatives to info dumping are to leave things as unexplained as possible and let the readers figure it out. Gene Wolfe’s Book of the New Sun has lots of names, items, customs and so on that nobody ever explains, but the book works nonetheless. Alternatively, work information into the plot as its needed (described here as infosalting) without overloading with too much info at one time.
That being said, I do think there are times an infodump really can work. Or at least not fail.
•Follow a good hook. The opening of DOA has Edmund O’Brien walk into a police station to report his own murder. A start like that justifies a few minutes showing his ordinary life before he learns about the fatal mickey someone slipped him.
•Make it emotional. That’s my strategy (which hopefully works) for several scenes in The Impossible Takes a Little Longer where KC explains her tortured past to her boyfriend. It’s a lot of information, but hopefully it’s a painful enough scene (this is stuff she’s really torn up about) that it’ll work. I’ll let you know how the writing group feels.
•Imminent doom. The dying man who gasps out a cryptic warning about the Old Ones. Or assigns everyone their quest seconds before a curse blows him up. Or spills the beans about why zombies are surrounding the house and trying to beat in. This adds emotion and keeps it from being a dull recitation of facts.

X-Men #12 is an excellent example of this. Alerted that the Juggernaut (much tougher in the comics than in X-Men 3) is attacking the school, Professor X has the team reinforce the defenses, then tells the story of his teen years and his history with Cain Marko. And all the while, something is smashing through the ice walls, the booby-trapped pit, the gas clouds, coming closer …
•The solution to a mystery. If the book poses a question, readers (at least me, anyway) are usually okay with an explanation of what was really going on (at least, most of the genre mysteries I read still do it that way).
Infodumps should be approached with caution, but they can be made to work.