I’m so old that I actually knew of folks who carried 3×5 index cards in their pocket, and every time they thought of an idea for something, they’d write it down. Then, they’d file those cards, and occasionally review those ideas for anything worthwhile.
I’m a big believer in capturing every idea, but my weapon of choice is Evernote. I use it on my phone and on my MacBook. You can snap photos, scan documents, drag stuff from email, and do audio recordings. And all of it is easy to search, so I don’t have to think hard about where to file it.
The tool isn’t critical, but find some way to capture every idea you have. Trust me, you won’t remember half of them. And most of the time, they’re not great. But it’s worth it to capture that one really, really good idea.