These tips are for designers, but some of them can be used by photographers too.
1. Read the site’s submitter guidelines (Shutterstock Guidelines)
2. Use proper descriptions and keywords – Use appropiate keywords to describe the submitted image.
3. Remove all bitmaps from vector file – Even if you didn’t used any bitmaps in your illustrations, after you save in an earlier format, some of the effects can be converted to bitmaps. Don’t use any Drop shadow or Glow effect.
4. Keep your vector files under 15 megabytes
5. Compatibility! – Save all your work in EPS file formats compatible with Adobe Illustrator 8 or higher. A file that is saved for a lower version of Illustrator is not backwards compatible and it will be rejected.
6. Close open paths (Try to use this script)
7. Submit illustrations with various subjects – The administrators of these sites want to see a variety of works, so don’t upload the same image with small modifications. After you get accepted, you can try to upload these images.
8. Don’t copy! – Use whatever image you want for inspiration, but never copy others artwork. It’s called stealing and it is treated likewise.
9. Remove all text – … or expand all text in your final file. There are several reasons why you have to do this. First of all, you don’t know if the buyer has the font type you’ve used. The second reason is, that you can’t sell others work, and all font are made by someone. So if it is necessary, use the system fonts and convert to curves when you finished working.
10. Don’t panic! – If your submitted images are not accepted, don’t give up. You just won another 30 days to improve your style, increase your portfolio. Maybe you could try another microstock sites, that don’t need sample files on registration.