@mensrea
Belgian law allows you to do what is called a "wilsverklaring" (declaration of desire) where you can say while you're still sound of mind that you want to be euthanized under certain given circumstances.

It requires that you nominate a person to make the final decision, and you still have to go through all the other (nontrivial) legal requirements, but it is possible and I know people who have done it.
@ariadne