I keep an ideas.txt on my computer (synced to my phone in Dropbox). When you're out and something strikes you (something frustrating? interesting? could be made better?) just jot it down. Capture it in the moment, don't wait until the end of the day (you'll forget).

Over time you'll see recurring themes (you get annoyed by the same issues, the same idea gets written down over several weeks) and it'll be somewhat self-motivating to pursue a solution. Your energy ebbs and flows but if an issue bothers you several times it's likely something you care about.