The blog iso50 asked a number of artists and other creative people what they do when the ideas aren't flowing. Here are three I like:
EVEN/ODD - Nicolas Felton alternates years: one year he travels more and concentrates on personal work, the next year he puts more emphasis on working and making a profit (obviously this one is an option only for free-lancers).
FEED THE HEAD - Khol Vinh does lots of reading and sketching. He says, "The reading part is a long-term strategy: constantly consuming ideas, influences, details, angles, metaphors, symbols, etc. and storing them in the back of your brain so that later on--sometimes much later on--you have a rich catalog of starting points to draw upon."
GET INSPIRED - Chad Hagan says, "If I'm ever in a rut, the best thing to get me out of them is to put myself in places that engage the desire to be good...get out and be exposed to others creating...going to museums, galleries, shows, etc."
DREAM YOUR WAY OUT OF IT - The agency Airside suggested setting your alarm for 5am. "When it goes off either get up and enjoy the unique feel of that time of day or go back to sleep and have the craziest dream (REM sleep is easier to reach/remember) -- one of these experiences will give you inspiration.
In the comments, feel free to add you own ways of getting inspired or over blocks.
(There's also lots of inspiration in my latest book, "Creativity Now!" -- get it from Amazon or your other favorite online or offline retailer.)