Split it between three places, in example:
- paper printed private keys stored in bank safe (don't forget to mention it in your bequest).
- QT wallet encrypted with strong password on offline computer, then uploaded to many different online storage services, emails, etc.
- brain wallet generated key with a very long but very memorable passphrase (i.e. password made with combination of very rare and private words like your mom's birthplace + your grandma's maiden name + address where your family was living in 1985 + your usual password).
A few SD cards or USB sticks will do
USB sticks often fail. A few corrupted bites and your wallet is useless. Besides having encrypted walled on USB flash, distributing it into many online storages will assure it's always safe and accessible.
Offline computer is only needed to create and encrypt your wallet - to avoid possibility that some virus/trojan steals your key while you create it.