1. Keep Contributing
I've noticed that when I upload more, I get more downloads. if I haven't put anything on for a while, my downloads drop off. Keeping a steady trickle of work going on there seems to keep the downloads coming.
2. Think Boring
The work that you like doing probably wont make the most money. Think about where your images will be used and play to that audience.
3. Think of a Niche
If you can find a topic that hasn't been covered much, then get in there and exploit it before everyone does. My most popular file is this retro robot illustration, when I first put it on there, I was getting loads of downloads off it. At the time, there weren't many similar files on there. Now there's loads, and most of them are better so I'm not getting so many hits on it these days. It's made me over $1600 so far though so I'm very happy with that file.
4. Strength in numbers
If the quality of your work is good enough to consistently get accepted, then keep piling stuff on there. The more work you have on in a variety of styles, the more chance you have of getting downloads.
5. Cross Promote
Make sets of similar styled and themed images and link them to each other. Make light boxes of bodies of similar images. People often want a bunch of similar looking images for a campaign and will download a whole light box of images.
6. Try different things
I will often try different styles just to experiment and to get some practice. Sometimes they go down well and sometimes they don't, but it helps you stand out if you are trying new things. You may come across something that turns out to be really popular.
7. Take your time
If an image takes you minutes to make, then it will probable be easy for any graphic designer to make. If someone can copy something and save the money then they probably will. If you've made a detailed, precise illustration, then people will download it because it's too much of a hassle to do it themselves. This circuit board illustration has made a nice profit, it was easy to do but it took a while.
8. If it's no good, don't upload it
If you've taken your time making an illustration and it just isn't working out, just delete it. There's no point putting it on and getting rejected, it brings down your approval rate (which you will been to become an exclusive contributor). Just start a new image and learn from your mistakes.
9. Be thick skinned
You will get plenty of rejections. Don't worry about it. It doesn't mean that your work isn't good, it means that it isn't suitable for stock. Illustrations for clients and illustrations for stock are very different.
10. Keep at it
Unless you get lucky, it takes a long time to build a portfolio that earns a decent income. Then just when you are getting somewhere, your downloads will drop from 10 a day to 1 every 2 weeks and you won't have a clue why. The earnings from stock illustration are very temperamental.