Keep your hand moving. That's what Lynda Barry advocates in her book, What It Is.
If you're writing and don't know what to say next, write out the alphabet. Keep your hand moving. Eventually the words will flow.
This works for drawing, as well.
I like to get up around 5:00am when the morning is silent, with a cup of coffee in hand. I read, or write, or draw. It's usually a creative time.
This morning my mind was blank.
But hey-- here's a ball point pen! And a scrap of paper! And look, there's a salt shaker!
And a pepper grinder!
I started drawing.
Boring sketches? Yeah! But who cared what they looked like? My hand was moving.
After a short break to shower and get the kids off to school I went back to writing. It turned out to be a productive brainstorming morning.
(And by the way, I highly recommend Lynda's book. It's a wild combination of graphic novel, memoir, and writing/creativity instruction-- very inspiring!)