Do you want the crypto nerd answer?
Timelock encryption
http://www.gwern.net/Self-decrypting%20files
Encrypt the private key using a timelock encryption algorithm so that it will take roughly 10 years to solve with a single processor (timelock algorithms are designed to make parallel processing impossible). Then as long as you are alive just move the funds every 5 years and publish a new timelock encryption problem. Make the timelock algorithm and encrypted key part of your will and when you die your heir can get the funds in 10 years by simply solving the latest problem.
* It doesn't have to be 10 years. Smaller is possible but requires more frequent changes and if there is a computing revolution or cryptographic flaw it gives you less time to move funds.
I like this idea!