When I only registred on Dreamstime in 2009, lots of my photos were rejected by inspectors. Shurely it didn't make me very happy. It went on for some time untill I understood one simple thing.
You should be positive on all stages - thinking what picture do you want to create, making it, writing keywords.
And one more - don't be hasty. In some days (2-3) you'll see them in new light and it'll help you select the best.
And don't forget about editorials - they are easy to get and can fit several situations. Something like that