You could send transactions to your heirs that would only be valid in, like, 2 years from now. Then, if you're still alive when these transactions are about to be valid, you just use the same inputs to make another transaction to yourself, paying a higher transaction fee. This will cancel the other transaction.
The problems in this are:
- You probably want to transfer everything you've got, including the money you move around all the time. So the delayed transactions would need to be canceled and resent every time you spend bitcoins.
- You probably do not want to send everything to one address only of your heir, since that would completely expose your finances on the blockchain (people would easily know how much you've got and to where you send your coins). The ideal solution is to make at least one delayed transfer per input you've got, what means that your heirs should provide you lots of addresses. Another way to solve this is by not sending the transaction to the network, instead, give it as a file to your heirs, and ask them to only send it to the network in case you die.
It would hardly be practical to do all that by hand. You would need some piece of software to help you.