MicrostockGroup

Personally I wouldn't recommend flickr...I've had problems with people downloading or copy and pasting those photos. I took the ones that matter off there.

Nataq's points are valid, but your friends/relatives won't be much use to you [1] unless they're already submitting at the agency/ies you're considering. I haven't seen the images Nataq says "you wouldn't believe your eyes what photos are submitted", and I'm not talking about these. I'm talking about perfectly competent ones which nevertheless have a few artifacts, or are taken in flat light (if that's normal in your area) or contrasty light (likewise), a logo which you need to zoom in to see etc.
Lots of the world's iconic photos wouldn't be accepted onto iStock for all sorts of reasons, and now I sit at camera club competitions thinking stupid things like "The light's too flat" or 'there's a shadow over the faraway eye of that eagle" etc, which are irrelevant in the real world.
So, everything that Nataq says, and then some.
There are many featured articles on iStock - you could usefully spend time studying them. Maybe some of the other agencies have similar, I don't know.
The NYIP website has some basic free-to-access 'how to photograph' articles which are very useful at the composition and technique levels in their ezine: http://www.nyip.com/photo-ezine/ezine.html and podcasts http://www.nyipodcast.com/.
Health warning: the hard info in these articles/podcasts is padded out by an insane amount of waffle. I used to copy and paste the text, cut out the waffle and keep the hard facts on my hard drive for reference. It's there, you just have to find it!
[1] Plus I doubt if I've ever had a friend or colleague not moving in photos circles who doesn't tell me "Your photos should be in National Geographic", so generally take their good opinions with a pinch of salt.
Another useful site is www.betterphoto.com, though nowadays they are mostly about plugging their courses, which I haven't taken, so can't comment on. They have free to access articles indexed here: http://www.betterphoto.com/allAbout.asp. I was an active member there for about a year before joining iStock. I'd say what was most useful there to me was recognising that American general taste for images is different from UK general taste. Since the US is probably the source of most stock buyers, that is very useful. They have monthly photo challenges, and the ability to feed back on each other's images, though it does tend to be of the 'wow, fantastic' variety. But overall if it's just improving your shapshots, that's a far better way to learn than iStock rejections, which tend to be very vague: my most frequent rejection nowadays is:
"We found the overall composition of this file's lighting could be improved. Some of the technical aspects that can all limit the usefulness of a file are:

-Flat/dull colors
-Direct on-camera flash and/or flash fall-off (bright subject, dark background)
-Harsh lighting with blown-out highlights that lack details and/or distracting shadows
- Distracting lens flares
-Incorrect white balance

For information about iStock lighting standards please see:
http://www.istockphoto.com/article_view.php?ID=524

For more information on iStock Lighting Standards, please see:
http://www.istockphoto.com/tutorial_2.2_lighting.php

Related Articles:
Lighting and Shadows:
http://www.istockphoto.com/article_view.php?ID=46
Setting up your own home studio:
http://www.istockphoto.com/article_view.php?ID=14
Custom White Balance:
http://www.istockphoto.com/article_view.php?ID=95

Decent Exposure:
http://www.istockphoto.com/article_view.php?ID=40

If you require further explanation regarding this rejection, please visit our critique forum for immediate peer to peer feedback. To visit the critique forum please see:
http://www.istockphoto.com/forum_threads.php?forumid=26 "

... which in my case always means flat natural light. So you can see that most of the links, useful in themselves, are of little use to help me to avoid similar rejections in future. (the only thing they could say is 'don't submit outdoor pics 95% of the time from where you live"). That's not to say that the articles on studio work aren't useful in themselves (READ THEM!). But don't imagine that any critique will be personal to your image. All the rejections are pretty general, with loads of links which don't apply to the actual image you got rejected.
The critique forum is good, and you will get personal critique. But @OP, to be honest, if you really don't care about whether you sell or not, iStock isn't the place for you. It exists as a commercial business to sell photos. There are other places to learn, even free/low cost. Maybe others can post here some examples of any I don't know about?

I dunno.  I keep seeing plenty of threads from newbies who "love rejections" because it helps them grow, and how great iStock is as a "learning experience" because they knew nothing about taking pictures.

well pointed out, Mr. Locke.
could that be because those who gave them this impression   were many years ago themselves clueless about photography and they were able to become "stock photographers" simply because  microstock was a fledgling , and
as you pointed out initially , Mr. Locke, it used to be easier to get their pictures approved . repeat "USED TO BE".

which is why there is so much emphasis every year on them buying next year's model to "make better pictures for micro stock" ?   

Good point again, that sometimes timing is everything.

I agree I wouldn't use flickr. Ever Tom, Dick and Harry puts photos on there and besides there is the problem with people cutting and pasting or downloading your shots, thats why I removed all of mine. Also if you plan to ever go exclusive with iStock you'd have to remove them anyway.

The real point is more along the lines that if you get rejected for your photo being too blurry and out of focus it honestly doesn't teach you anything. It tells you that you're image is blurry and that's all.

It doesn't teach you about glass quality, focusing or operation. Was the blur created from using a soft lens that will never produce micro-acceptable images to begin with? Did you simply focus on the wrong area in the frame given your subject matter? Were you maybe shooting at 1/60 on a 300mm telephoto at full zoom? Were you shooting a single subject too close with an aperture of f/1.8? Were you shooting multiple subjects too close with an aperture of f/5.6? Should you have braced your body during the shot? Should you have held your breath? Should you have used a tripod? Should you have altered your shutter speed? Should you boosted your ISO (which I never recommend)? Changed your aperture? Switched to an appropriate lens? Added another light source?

The rejection notice didn't teach you any of this and it is not designed to teach you this. Instead of uploading thousands of bad images and expecting to learn from it people should focus on learning how to take a good photo in the first place. Until then they are simply not ready. Microstock is essentially a career. Regardless of how much money you make, you are trying to produce income therefore it is a job. The goal of a job isn't to make a couple bucks and waste a lot of time doing it.

Shooting, keywording, managing and uploading takes a lot of time .. why waste that time when you could "seriously" be learning?

Thank you all for your comments!
You've all given me much food for thought, and awesome tips that I will definitely try out soon!!!

Well, I am glad I initially thought of microstock a good way to learn, or else I would have never stumbled upon this website which has such a wealth of information and learning tools!!

If you can pass the initial photo evaluation, I would say Shutterstock...but if you decide on another agency, I would log into the Shutterstock forums.  They are really helpful!

If it's really only the one then I would go with iStock. assuming that you're good because they'll give you a really hard time if you aren't. And if you do Ok you can always go exclusive a little later down the line.

If the prospect of iStock sounds a little duanting then I'd opt for shutterstock. By far the fastest turnaround and not so picky once they've accepted you.

 i have basic kit  cannon450 and mainly shoot with 55-250IS ,started last april ,   when i submitted everything on hard disk, soon realised i was wasting my time, i shoot mainly for rf123 and bigstock  and have 59 and 21 images accepted respectively,  3 accepted with dreamstime ,  and a total of 4 sales(witwoo) 3(rf123) 1 (dreamstime) and use rf123 and bigstock  as quality control, 
id love to produce a photobook of  my sales,  lord knows how long that will take,    but my local college are offering free digital course 10 week, i start thursday,  its very fortunate i have a day job,  but i will keep plugging away

From my experience, a few months after joining this forum as a newbie, you have to aim for IS and SS

I started out by contributing to the low earners until I was confident I could meet IS/SS standards. I am now signed up with both and, to give you an example, the 9 images accepted by SS as part of my application generated 11 downloads in 24 hours. Those same images haven't sold once in 3 months on low earner/new sites.

I'd say SS. Cos you get results very fast and it motivates you further. If my pictures didn't start selling there I probably wouldn't stay in MS business. Other sites just took took too long to get going.