I'm relatively new to stock. After sending more than 250 files, I've finally stepped over the 100 accepted files barrier. Considering most of the first ones have not been shot as stock in the first place, this is not such a bad start. My ratio has been increasing steadily and I understand the whole concept better now.
Here are some tips that beginners should find useful:
1. Always upload your best work. Be critical of yourself and filter out those average shots. This will keep your ratio up and allow you to upload more and be higher up in searches.
2. Do not upload too many files from the same series, or files that could easily replace each other from the designers' point of view
3. Tag responsibly and to the point. Leave it to the designer to figure out what the image he's looking for should contain.
4. If you're investing your time and money in post-processing and/or lighting, do it right the first time. Get the most out of your pictures. Pay attention to details such as noise, dust and excessive sharpening. It's a shame to get a good picture rejected because of avoidable technical issues.
5. Lastly, when shooting, think of the possible uses for the image and the feeling that you would prefer the 'finished product' to have. This will alow you to be more creative on the spot and capture a more interesting image than just shooting e.g. from a random angle.
Hope this helps. Please share your experiences.