You basically delete the private key and won't be able to sign a transaction with it ever.
See http://moneyfactory.gov/uscurrency/damagedcurrency.html There's also an article with some interesting stories: http://usatoday30.usatoday.com/news/nation/2001/06/2001-06-0...
[I'm not intending this as an argument about Bitcoin, but as something people might find interesting.]
(Or at least that's what I was told during a tour of a US Mint several years ago.) Money burned up in a fire is not replaceable.
And this is one of many reasons why it will become increasingly important to beef up the security and robustness of the Bitcoin network and ecosystem. It has some of the same failure modes as traditional bank/government-backed currency. But it also has a set of a different kind of failure modes that should also be minimized or mitigated.