Now it would be nice if you could upload your images to the microstock  sites, sit back, relax and watch the money roll in. Alas the world is not always a nice place and to really make a decent income out of microstock you will have to get yourself seen.
Also read my article on setting up a photo portfolio space . This article is an extension, on how to promote that profile space and use it to send potential buyers to your images at a microstock agency.
Getting yourself 'all over the internet'
The problem with this is 1) measuring the results of all this and 2) the amount of time it takes - is it worth it?
90 - 10 principle comes into play, 90 percent of your work/sales is brought in my 10 percent of your 'promotion efforts' (in fact it might be more like 98% -2%!)
Link to your portfolio site or microstock referral address on the bottom of your email signatures, your signatures on forums that you are posting comments on (keep those comments useful and professional, and NEVER post something like "hi, great photos, look at my photos on myphotolalala.com"). Generic spammy comments might be tempting but it will get you banned from sites, upset people and also makes you look unprofessional.
Write Articles and Syndicate
If you are so inclined and your images suit the topic then write an article about it for your site and syndicate it for others to use. Websites such as articlesbase.com  allow you to distribute your article allowing other websites to display it if they include a link to your website. E.g. "Experienced Chef and Food photographer Joe Smith looks at recipes for...." The list of ideas is as the saying goes is 'limited by your imagination', you don't actually need to have taken images related to your article, but your article will be featured on a site relating to your article so it's common sense that if someone clicks on your link they will be interested in that topic and photos relating to it.
ezinearticles.com  is another site where you can post and article. You should ideally look at guest blogging and strategic partnerships as a more professional alternative to syndicating articles via article marketing services.
Flickr and Smugmug and Webshots and...
All of these websites are more than just a way to upload images of your car, cat or kids*, they are a great place to self promote, there are a lot of image users out there who are searching flickr right now for images that are free for them to use under a creative commons attribution license. Now imagine you post some of your images for free on there, a prospective buyer sees something they like and clicks on it, on the site bar they see several of your other images that they like even better (images which you have maintained full rights over and are not provided for free) they check out your profile on how to contact you and see that you have linked to one of the leading microstock sites... this can now go one of two ways...
- They have purchased from the microstock site before so they follow the link and just download the images.
- They have never used a microstock site before so they go check it out, because they liked your image so much they signup and buy the image.
- They are determined not to pay a penny for an image, so they use one of the free images and as per the license they credit the photographer and the website it is on as the source with a back link (ideally).
Either way you have come out on top (minus the expense of time in uploading all those images). People won't just click on your referral link, sign up to a site or buy your images out of the goodness of their own heart, they buy because they have a need --- one which you should be able to fulfill.
*in fact I strongly advise against mixing your personal images and your stock images, but that is not to say a blog about your stock image work can't have a personality and include background material.
Making use of rejected images
Images rejected by all the agencies? or not quite good enough to upload? They are not 'dead images' but a valuable resource, try uploading some to sxc  or to other free stock photo sharing sites like those listed below, perhaps at a lower resolution. Images of say 2100x1500 approx are not too low resolution, designers are still looking for usable images on these sites and might skip your images if they are 800x600. The trick here is to make sure that people are seeing your profile on these sites, and that it contains a link to a portfolio on a microstock site / referral url / or a home page you might have somewhere - even a flickr profile etc. 'Buyers' on these free sites can be converted into image sales, it only takes one user on a free site to see your referral link and then click it and sign-up, even if they don't buy your images you receive a commission on the images they do buy.
flickr  (as a creative commons image)
sxc.hu  (perhaps already saturated)
It's important that any site you do upload to allows you to include some kind of link to your paid microstock images. Does anyone else know or have results from other sites? It's difficult to measure how much traffic offering these free samples generates, and how many of those visitors can be converted into useful buyers. For some photographers 'free rejects' could do more to damage their brand than promote it.
This option is only really practical if you have some sort of specialist service, or something newsworthy to write about."I've just uploaded some images to xyz.com" is NOT a press release! It really has to be something that is useful or interesting to the general public or a large market segment, like exclusive images you have made available that are also on show in a local gallery, or quality images and editorial from far flung locations that you have placed on your site as a teaser for a book you are self-publishing. You would write a press release if you had worked on a project for at least several months and received some recognition about it (award, gallery viewing, commendation from a professional body etc.)
Blow your own trumpet if you think you deserve it...
Entering Competitions is a great way to get yourself noticed as a photographer (if you win that is). If you have won a high ranking competition or had your work published in a well known magazine it will make your work appear more credible. It only really works for promotion of your microstock sales if you have your own website or blog which you can use redirect visitors who have been interested enough in your competition entry to look you up, or you can post your successes in a 'Testimonials' section. It's difficult to measure the advantages here, but just one competition win can get your work seen by someone important that opens a new door; on the flip side it might just all be a waste of time! I'd recommend you check out pro-imaging.org  who have a photographers "Bill of Rights", they review competitions which often take away rights from photographers who enter them. Take great care to read any terms and conditions on anything you submit images to, or make sure that the competition you enter is on the pro-imaging "Rights On" list.