Whenever I write something down that I want to remember for the rest of my life, I always forget about it. What should I do?

There is an assumption in the question that writing down something means that you will recall it for the rest of your life. Yet you have tried this and then have difficulty recalling it. However it would seem that you need to do something extra with the writing down so that for the rest of your life you can recall it.

For something to be recalled on demand requires an intense emotional desire and focus on the something along with a general association that takes you to the memory of that something. For example if I say ‘fortytwo’, it is an association to hitchhiker's guide to the galaxy by Douglas Adams which can then help me recall many memories from the radio series and books.

Writing down in what I can only describe as ‘empty space’ that you have no memorable associations surrounding the thing you are writing reduces the ability to recall. A mind map, as described by Tony Buzan, can help you understand how you associate and structure the concepts of the thing you want to recall. However any boredom and or emotional pain you may feel doing the exercise of creating a mind map will again make recalling difficult. The brain likes to protect us from pain. So if the process of noting down a thing is emotionally difficult then recalling feels like forgetting.

People tend to recall in detail things they are intensely interested in.

So you can try to find the laughter, make an interesting mind map, look for the why you want to recall, and find as many associations you can, these things should help in getting you the recall you desire.