The bar is continually rising, it's getter ever harder to be accepted as a photographer by the top microstock agencies .
Signing up to sell microstock is free, but that does not mean everyone can do it. Most agencies vet their photographers before they are allowed to upload in earnest. Most sites make you submit some sample images and/or pass a multiple choice test on their site.
Completing your Microstock Profile
One of the first steps to a successful application is completing your profile at each agency you apply for. I'm not saying that all agencies take this into account when accepting a photographer, some might not even look at it. A lazy, incomplete profile with missing or incorrect contact details will not exactly scream "I have high standards".
You should be prepared to complete as much of the information as you can, your equipment, and include a creative statement about your photography and why/how you do it. You can include a self portrait (or avatar) and in some cases link to a gallery website with more of your work if you have one. This all helps enhance your online presence. Although most people start in microstock as amateur photographers, all of this information makes you come across as a little more professional in your outlook.
Passing the Photographer Test
The test should be fairly easy to complete if you have read up on each sites photographer guide. Most of the question's in these tests are common beginners pitfalls. Questions can be easily answered if you take the time to read the photographer hand book or standards guide at each site. At some agencies if you answer incorrectly, the site displays the correction and you move onto the next step. The test is a learning process for you and a way for the agency to weed out people who don't really understand what stock photography is about. There are usually some questions on Intellectual property issues  (trademarks and logos etc.), Model releases , and some obvious questions like "Do you think it's okay to find someone else's photo on the Internet and upload it for sale?" (I hope you already know the answer to that one!)
Sample Stock Photos
A new agency may also ask you to submit some sample images. These may be more critically vetted on technical correctness, but should also be useful as stock photos. Stick with conventional, 'typical stock images' and don't submit anything that's too outlandish. I have always submitted a selection of images that in some respects looks almost like 'stock clichés' but these seem to go down well for evaluation. The times I thought that an agency was looking for 'edgy and trendy' and I submitted my grungy and artistic photos I had my fingers burnt. You should check every image thoroughly before you upload it, and in the case of your sample images make double even triple sure that they are perfect. If there is anything that you don't quite like about the photo, something that you would change then that's not the photo you should be using as a sample.
Choose between 5 and 10 (this varies by agency) of your very best images. Each should be of a different subject, If you can show each different style of photography you have mastered (night time, portrait/people, scenic/landscape and tabletop/still-life/studio) then that is also a bonus. The samples must be perfectly focused and exposed, and here again be conventional - a wide depth-of-field with some highlights and shadows is better than high key and narrow depth effect.
Avoid including images of:
- Family pets / cats and dogs
- Sunsets, especially where the sunset is the only subject
- More than 1 landscape/scenic image
- Motion blurred images no matter how interesting they look
- Abstract light effects and blurs unless they also serve a concept
If a site does not request sample images, then upload your selection of samples (5 to 10 images) as the first batch of photos you submit. Only one chance to make a first impression, and at some sites if you fail they make you wait for several weeks or months!
More reading: What kind of images do microstock agencies sell the most 
Stock Samples: Nine of my sample images I choose from to submit with my applications.
Note: It was difficult to decide if I should show the above samples, I don't want to let these images affect anyone's choice of samples or use them to decide if their own work is good enough. They are presented here in the spirit of openness that microstockinsider embodies. Everyone's photography is different. I should also point out that these photos were acceptable in 2005 and onwards by what are now the top microstock agencies, and more recently by some of the newer sites. These images might not be acceptable now as standards are constantly rising at the top agencies.
What if I fail?
Hopefully the feedback will be good and you understand where you went wrong. It's annoying if they say "not quite what we're looking for at present time". If you are rejected I'd suggest further reading in 5 common reasons for rejection , that post is aimed at photographers already accepted but the process and reasons for failure are often similar, we have written a few suggestions on how to deal with each scenario.
You could try a few of the smaller start up agencies and see if you can sharpen your teeth there. We have reviewed a lot of sites for microstockinsider, there are limited earnings to be made on the sites lower down in our top ten , these are by no means 'easy options'. Your eventual goal should really be to have your images accepted at each of the top 5 agencies.