My personal system for at least ten years.

1. Capture everything

The first and most important step is to get ideas out of your head as soon as possible. Write things down as they occur so that nothing is forgotten—in practice this may actually help you to remember.

2. Organize if needed

Move it where you are likely to encounter it. Go through old entries periodically to find potential connections to newer content and bring it into the fold.

3. Purge

Do it or delete it as soon as possible.

Prune regularly as a way to lighten the load and to synthesize old into the new. If it's been there for 𝑥 weeks, it will probably never leave, delete it.

Delete can mean 'I don't want to be reminded of this'.