Clearly most people can choose to end their life, but what about people who cannot? If you have locked in syndrome where you are concious but unable to move or communicate in any way, or even if you can communicate but are paralysed, then you can make the choice to die, but you cannot action it.

I suppose the way I would approach assisted suicide would be;
You must be of sound mind.
You must be able to clearly communicate your intention to die, but prior notice is void unless you are no longer able to communicate.
The prospect of any remission of a medical condition must be so remote as to be considered impossible.

Im not pretending I have this at all correct, and I certainly would not have the confidence to make the rules in real life.