I've been seeing a lot of people on twitter lately who log on and spam links to their own items. Honestly, I scroll right past all of them. I don't mind someone linking to their items once in a while, but if all I see is 20 posts with links to 20 different items, I'm going to scroll past - and I know I'm not the only one.
Twitter is designed for networking, letting your customers know who you are, meeting people with similar interests. If they're interested in you, they'll click the link on your profile and the links you post once in a while - if all you ever do is spam them, they'll either unfollow or mute you eventually, something I've been doing a lot of lately.
So what should you do? Post what you're doing in your everyday life. Not every little thing - but things that people might be able to relate to or find interesting. If you found an interesting article on something - business related or not - post it. If you tried a new recipe - post it (if you found it online or blogged about it). Did you learn something today? Did you discover something new? Is there something new in the market that you want to buy? Post it! Seriously, anything that interests you likely interests someone else out there.
Also, follow people and groups that interest you - your favorite fan websites, favorite artists, or just people that have similar interests to you - and RT whatever catches your eye.
It took me a while to figure this out, but since turning over a new leaf (twig?) on Twitter, I'm seeing more people checking out my shop, and I can only assume that it's because other people, like me, just scroll past the majority of "check out my new item" links posted. I still post an item once in a while, and I still link to other peoples' items that I find interesting (putting an @ in hopes that they'll return the favor), but I also write about my favorite games, technology that I want, TV shows I'm watching, my son's antics.
And since I've started writing about all that stuff, not only are people paying more attention to what I say (and clicking links more), they're also responding more - and I love it when people actually respond to my comments and we can get a conversation going.