Some additional things you should know:
#yalitchat happens on Wednesday evenings, and is a LOT of fun with writers, agents, and readers. Just search #yalitchat and you'll see everything that's being discussed. If you want to talk to somebody so that everybody can see, use the "mention" or "reply" feature. If you want it to be private, DM (direct message) them instead. I find this particular chat to be fun and helpful, but there are other chats, too. Usually, you can find them via writer and industry blogs.
The great thing about joining a chat-in-progress is that there's a steady flow of conversation and ideas, and you have a chance to be witty and quippy. People usually add you as a friend if they appreciated your contribution.
As far as garnering followers, I'm still figuring this out myself. Joining a chat seems to be a great way. I also retweet things I especially enjoy, and often the person I quoted follows me back.
Twitter etiquette: never reply to a stranger or DM them just to give them your links--unless they asked for it. :-) It's a big turn-off, and I've unfollowed people because of it. There are plenty of opportunities to tweet your links without this, anyway. The people who do this usually don't get many long-term followers. If somebody mentions you or retweets you, it's nice (but not required) to thank them, either publicly or in a DM.
All along, you'll get "spam" followers--people who have nothing to do with you or your tweeting subject, but just want you to buy their products. Just ignore them, unless they're offensive. Then I block them. If you ignore them, they'll probably unfollow you, but you're better off without followers like that!
If someone follows you who does share your interests--FOLLOW THEM. They're more likely to take an interest in what you're saying when you reciprocate.
My favorite published authors and agents on twitter are the ones who are friendly. They respond to a question of mine, or ask questions of their followers. e.g. "Writers, what would you like to know about the publishing process?"
The best way to use twitter is to correlate it to your blog. Whenever you update a blog post, tweet it--that's the obvious one. But people are getting really creative with it, too. You can have a twitter contest. The author of Gimme a Call started a twitter revolution when she invited tweeps to write letters to their high school selves in 140 characters. My fave was "Dear High School Self- Put the tweezers down! #gimmeacall." By getting everyone to use her title as the hashtag, people were (sometimes inadvertently) advertising her book! Publishers have had contests to win free signed books. One asked people to write a rejection letter to an author in 140 characters. Just make sure you tell them a hashtag (#) to use so you can search the responses!
Lead a discussion using the hashtags, too. Talk about what you #amwriting (a popular hashtag). Or make up your own hashtag and invite readers from your facebook page, blog, and twitter to contribute to a groupthink.
Those are just some of the ways I'm seeing twitter used successfully.
Follow authors (we always follow back), agents, editors, publishers (they are coming to twitter in droves). When you are looking at somebody's profile, you can see their LISTS on the side of the page. Agents often make a list of their authors, so you can actually click "follow this list" and now you have access to all of their tweets. I've done this on some aspiring authors' profiles who had an AGENT list. It's really helped me to discover more agents, which will be good when I'm done with my WIP and ready to submit. You can create lists, public or private. I keep my family list private, but my "Book People" list is followed by a dozen other people.
Find me on twitter: @Katrinalantznov