Thanks for the message. I think the most important factor is that you need to have work that people want to buy and that the stores know will sell.
Another key aspect is association, you need to get your work seen with the right brands and with the right people to make it desirable.
Try to get stocked in a few small stores or get your work blogged about and use social media such as Twitter, Flickr, Pinterest and Tumblr to advertise your products.
It takes a bit of patience and hard work, U.O got in touch when someone at their head office purchased one of my prints, they showed it around and then I was contacted with the offer to sell through them. I've also been talking to Paperchase recently via a publishing agent in regards to stocking my greetings cards in their stores. It's difficult to get items stocked in large stores like this without the help of someone, be it an agent or someone who works in the store.
Good luck with everything!