Go to a local area. Take one lens or restrict yourself to shooting at a chosen mm on your zoom lens. Spend an hour shooting whatever you see around you, without too much thought about "Is it stock?" or "How's my composition?" or "Are my settings correct?" You will probably end up with a lot of crap, but you might get one or two images that surprise and inspire you.
Go to the bookstore. Look through photo books and magazines. For illustrators, peruse the children's books! Take a sketch book or notepad and jot down descriptions (or sketches) of images that appeal to you.
Take the sketchbook or notepad you took to the bookstore and expand on those descriptions. Let your mind roam free!
Grab a dictionary. Pick a word at random and brainstorm about images that would illustrate that word.
READ! One of the best places to get ideas is from fiction. If the author is good he/she will describe a situation or object well enough that you can conceptualize it and put it onto film or paper.
Take a walk. Better yet, take a walk with a dog! My dog like to stop and sniff every few feet. Which, of course, means I have to stop too. Rather than get annoyed, I try to take the moment to look around me and see if there's anything interesting. Sometimes you can find very interesting perspectives of places you would have never, EVER stopped.
Go through your junk mail. It's advertising, ya know! You might find something there that inspires you -- or maybe something you or a friend created!
Join a camera/artist club. If there's a good mix of people you should be able to get some very different perspectives on the art you are involving yourself in. (Remember, stock and art don't necessarily mix - but sometimes you can get "stocky" ideas.)
Once you've been inspired to work again you can use the ideas you gained using any or all of the above techniques. But, and this is important! -- DON'T COPY. Take the inspiration you got from an image and make it yours. Change the color, position, object, concept - something! Straight copying isn't the way to break a creativity block, it's just the easy, cheap way out.
Hopefully you've gained some inspiration from this blog ... I'm feeling a lot less blank myself so off to work go I!