Whenever I feel in a funk, I usually start with a cup of coffee. If that doesn't work, I will look through my massive collection of art magazines, pack up some colored pencils and paper and head to a book or coffee shop, browse blogs, browse Etsy, or look for more interesting illustration friends for my illustration Facebook profile. Usually, just finding a source of inspiration is enough to get me back on track. If that fails, sometimes I open a random book or magazine and blindly point to a word - whatever it is I draw owls, or cats, or fish (or whatever I think of) doing that. So the last time I did this my sketchbook ended up with: Local Residents, Owl Mixer, Fuzzy Owl, Garden Owls, Main Unit.
If its one of those clunky days when everything looks ugly and it seems like I have 2 left hands, I usually change mediums and just have fun doing something totally random. (like a collage, or a sharpie marker drawing, or a graphite version of my owl, or sculpting an owl out of clay).
I also agree with Cindy - utilizing a blog and then participating in challenges from other blogs is a great way to push yourself. Monday Artday is another blog I'm part of with art challenge topics.
This was a piece from my Illustration Friday challenge "Capable:"
http://www.etsy.com/listing/97075079/almi-art-preparing-for-lift-off-original