I do pretty much the same thing Cuddlesweetdream wrote, I tweet about my shop but also mix it up with different venues, i.e. cooking, wine so my followers are not disenchanted with a bunch of spamming.
I also add users that are relevant to my shop, I use alot of copper findings in my jewelry, so I have many copper piping and manufacturers as my followers. I have never had a sale from twitter, but have lots of views when I post a new item.
i mostly use twitter as a connecting portal to all of my social networking sites, Facebook, Blogger, and Etsy.