Sometimes an idea will approach you when you least expect it. Always keep a sketchbook handy or maybe even just a pen. Grab a recycled piece of paper, cardboard, or whatever scrap and be sure to mark down the idea. Sure you may not be able to recreate the moment down exactly how you first envisioned it but capturing the "idea" is what's important. The image below is an idea I got very early Sunday morning, now only one hour is left in the day and I finally completed that thought. Hey life happens so sometimes you really just have to keep swimming (a quote from Finding Nemo, if you didn't know, a favorite animation of mine).
A movie poster based on my son, Max.