Piggybacking on the excellent blog article by Ellen Boughn about keywords, I thought I'd share a technique that I use for keywording when inspiration runs low (probably because I used it all to shoot the pictures, lol!) and that may or may not give you a hand if your "keyword ideas" tank is empty.
You should have some keywords about your image to start with. Even when inspiration is lacking, I know you can come up with some ;-)
Use the keywords to do a search, rank images by relevance, and open the detail page for the images that most look like your image (or convey the same idea).
If you look at those images' keywords, you have a pool of keywords to choose from.
Certainly not all of them will apply to your image, but going "yes, no, no, yes" from a list is much better than getting frustrated because the ideas don't come... It's good to keep it in mind and use it as a "backup solution", but I would not use it as my main keywording technique.
Obviously there are other (more creative) techniques for finding keywords that apply to an image, and I will probably write about them some time in the future.
Good luck with your keywording...