Some people go so far as to engrave their Bitcoin address on steel plates.
Steel melts at around 1370 degrees C (2500°F)

The average house fire burns at a temperature of about 1,100 degrees Fahrenheit, which isn't hot enough to destroy most metals and earthly-made substances. And if an item is well-placed and small in size, its chances of survival increase drastically.

or you can go totally hardcore and get a Bitcoin tattoo. That may be a bit creepy though, like a scene from The Book Of Revelations, but you'll never lose it.