I use my peripheral vision to see things. I let things creep into my brain from the side. I find that the stuff I think I'm seeing is actually more interesting than the stuff I'm actually seeing. So if I see something cool out of the corner of my eye, I linger there a moment, ask myself what it might be, then I let myself look at it.
The asking of what it might be allows me to hang onto that creative moment even when it's over. From there I may sketch what I saw. That sketch might turn into something, or it might not. That part doesn't really matter so much. It was the openness—that's what mattered.