What's the most foolproof way to ensure your bitcoins can be passed on safely to your heirs in the event of your death? Note this question has two levels of difficulty:
safely passing on bitcoins to someone and
passing them on according to the terms of your will, which may split the "bitcoin estate" two or more ways, and not necessarily in the same proportion.
Point 2) implies giving control of the wallet to a Trustee in a sufficiently transparent way that the heirs can check the wallet amounts to ensure they receive all the bitcoins available.
A complication I'm aware of is that as I'm using my bitcoin wallet, any transaction wipes the existing key and transfers the balance to a new key. So storing a paper wallet is not really an option unless I intend to never touch the bitcoins in it during my lifetime.