I stumbled across this monthly book cover design contest last summer, when I was making my own cover and feeling very anxious because I'd never made one before. Authors submit e-book covers every month and Joel Friedlander (read about him here) makes comments on each, explaining what works and what doesn't and why. Even if you're not daring enough to submit your own cover efforts (or maybe you are!), looking through the contest entries and reading the reviews might help with your design. For instance, you learn quickly that red lettering on a black background does not usually work well. Check it out and get some book cover inspiration. Then, use the free GIMP to make your own.