These are all great suggestions, but the best answer sort of does depend on the story.
I have this method is to grab an image from NASA's astronomy picture of the day page. A lot of these images are copyright free. Then I throw a title over it using MS Paint and a fun font from Blambot or a 3D font using the fontwork function in OpenOffice Writer, and maybe a simple illustration. I might call this the Bob Ross method for sci-fi book covers, a way of making images which looks much harder and more time consuming than it is. I've been pleased with my results, but I write space opera in which this trick fits well. This wouldn't fit at all for most stories.
If in doubt, ask an artist for help.