I come up with ideas in the strangest places; often when I’m alone or performing some other mundane task. One thing that I often do is immediately write the idea down in a notebook or my iPhone/iPad. My memory isn’t perfect and in the past I frequently struggled to capture the essence of my brainstorm; but I often got it wrong or at best half right. It’s important to capture the initial shape of your idea at the moment it pops into your head. Did you write down your last great idea? I did.
If you want to go old school get a moleskin notebook and use sticky pads to manage your thoughts. All inventors keep engineering notebooks.