That's a tough one! It's something I've struggled with a fair amount lately. The most important thing to me is to always be ready for when it strikes. I get ideas at the most random, and often inopportune times. I am quick to write them down, because much like dreams they have a way of evaporating without your intention.
Another way it often strikes is when I'm listening to music. It has to be good music, my favorite stuff. I'll close my eyes and let myself be immersed in the songs and enter a trance-like state. Ideas or images float into my head, and I know they are some part of my subconscious mind, but it is as though they just show themselves. A lot of times I jump up and grab a notepad and start drawing at this point.
Finally, entheogens are seemingly an endless well of inspiration. I don't usually try to draw what I see. Maybe I'm just not practiced enough at drawing, but it seems like when I try to draw tryptamine/mescaline images it never works. It's like the harder I try, the harder it gets. So, often during a trip drawing is fun, but my real bread and butter is the after-glow. During the time directly after trips, and the next few days is often when I've come up with my best stuff.
So there you have it! Mr. Peabody's own confusing, murky, completely unguaranteed path to inspiration!
Be an adult only when necessary.