I don't know how to be successful at it other than to take the kind of pictures I don't want to take (people doing work/office-y things). I hear that's one of the most popular categories.
I do not recommend Acclaim Images. There was some sort of big backlash after they took down their window into who's downloading your comps. I finally deleted my images and quit, and after at least a month they still haven't deactivated my account and I'm still on their stupid email list.

They used to have some really good people working with clients and also available to work with artists but they very suddenly got rid of those people awhile back.
I tried to sign up with absolutestockphoto.com (twice) and they never replied. I emailed them again to follow up on my submissions, and they didn't reply to that, either. They could at least tell me we got your application but have decided not to accept you, as their web page claimed they will.....