The question has been discussed on here before.
I believe the easiest way to do it is to have the details formalised in your will, but since, as Fritz suggests, things are changing fast in MS, you should take your lawyer's advice on how to word it, so that whatever agencies you might be on at the time of your demise will be covered. Otherwise you could be dropping in and out at a regular and costly basis.
You should probably also leave your access details, but hopefully as long as you've taken care of the details in your will, the agencies should be able to work with your beneficiaries, as you may have to change passwords etc from time to time.