Be unique and memorable. Write about a variety of things. Include little tidbits from your life. You don't have to be too personal. I cracked up recently over a Tweet that was raving about his delicious dinner. You can stick in people's minds by making them think, making them laugh, or interesting them in something scintillating. Talk about your handmade business, making crafts, your crazy kids, your crazy dog, anything you like. Share your favorite quotes, words of wisdom, or fascinating trivia.
Post regularly. There is no rule of thumb, but between several times a week and 1-3 times a day seems reasonable. Don't overwhelm those people who aren't following many users. Remember, people will stop following you if you post too much. People will forget about you if you post too little (this is my personal problem...).
Interact with others twitter users. Social networking is called "social" for a reason. You can stay in a bubble, but it's more fun and effective to make friends.
Add contacts on twitter. These can be people who follow you or those you find interesting/part of the target audience. Many people use websites that gather more followers on twitter. I'm slightly suspect of these sites and people who use them. However, these people have reported great results. Timothy Adams gives a good overview here about these types of websites.
Please, oh please don't just post about your latest blog entry, listing on Etsy or Artfire, or new treasury. I know you're proud and want to promote your business. I'm proud of you too. However, most people don't want to read links. That being said, Twitter can help promote your business. Many people on Etsy forums report getting 20-30 more views when they twitter about an item. It should be noted that many people reported not seeing any spike in sales from twitter, only an increase in viewers. Just don't let your business be the only subject. I think sharing bits and pieces of your life as an artist is natural and useful in letting people get to know the person behind the business. The personalized approach to the handmade business is one big reason many people shop at Etsy and Arfire.
Let Twitter express your brand on a day-to-day basis. Remember the list of adjectives you made back to describe your handmade brand (see this post)? Keep those in mind when twittering. If you sell children's clothes, maybe include more posts about your kids. If you sell nerdy, geeky items, write a bit about the Star Wars marathon you had last week. Target your posts to be to attractive to the ideal customer. Chances are, if you enjoy creating your products, twittering in that "theme" shouldn't be a problem.
When necessary, use hash tags (#) so you can be found in Twitter's search engine. Some useful ones to know are #etsy, #handmade, #myetsy, #artfire, #giveaway. The hash tags acts as a little stop sign for Twitter, "Stop! Look at me" so when people search for the above keywords, they just might find your little corner of the world. Remember: too many hash tags can be obnoxious. Use only when the tags add value to your post. Concentrate first on sharing information, knowledge, daily life, etc.