I second the suggestion of an idea-list that you keep on hand-- but also: Brain-dumps are great!
Set a timer and just write/type. Let your brain fill the space and write the words as your thoughts wander- it could just be about how your day is going, a recent interaction with a stranger, or a food you're in the mood for. Just write to write and let your mind wander-- same with art blocks! set a timer and just put some vague lines down.
It won't immediately give a "eureka!" every time, but it's a lot like doing stretches before a more strenuous exercise- I highly recommend it 