Others have mentioned a will and that is a good idea. But consider putting the private key and detailed instructions in a safe deposit box and then reference that safe deposit box in the will rather than giving the private key or clues to the private key in the will.
Yes, it's a good idea. Just write all your passwords, keys detailed on a notebook and keep it safe in a place where just who will get your heritage can have access. Maybe a bank or a safe box. Or give it to your family members, so they can use your BTCs if you unfortunelly die.
yes that is right to say, but if you are giving it to some one in advance then there is a chance that he may use his private key in advance. i think the best option will be to write the key on a paper and then keep it in a safe place and write a well in which he will give some clue to a person whom he want to use his key after his death, so this way his key will be safe.