Microstock, like most things, disproves the old saying, "Build a better mousetrap and the world will beat a path to your door." (Ralph Waldo Emerson?). If you want to make a sale, you need to get the image in front of people's eyeballs, and to do that, you need to find as many relevant keywords as possible. The more searches that hit your keywords, the more that your image comes up in front of designers. (Note that it is important not to "spamdex" your image in an attempt to get your image viewed using keywords that don't match the picture. This is grounds for expulsion on many microstock sites, and is not very courteous to the designers). Below are some tips on keywording--feel free to add more in the comments.

1. Come up with as many keywords as you can for an image by looking at the image and describing key elements as best you can.
2. Consider looking up important keywords in a thesaurus and adding relevant alternates to your list.
3. Get tips from others: search for similar images on various microstock sites and examine the list of keywords that come up on them, looking for relevant keywords that you have missed. (Note: don't copy and paste someone else's keywords).
4. Aim for 25 to 50 keywords per image (it will be hard, but worth it).
5. Include a title and short description.
   
6. When you are editing your images (e.g. in Photoshop), add the keywords directly in the EXIF data. This way you only have to enter them once and even if you upload them to multiple sites, the keywords will be extracted from the file automatically.