1. Establish and build your brand.
Whether you realize it or not, how you interact with people on Twitter, the content you share, and your profile and header images create a brand that's attached to your name. You can create a brand that will help you sell books. So, think about your target audience. Do you expect moms to buy your book? Cultivate a following among mom bloggers by writing and sharing content that interests them. Releasing a steampunk novel? Develop a brand that attracts readers of steampunk books. Tweet about steampunk events, topics, and books.
2. Share the love.
You don't have to follow every person who follows you. (However, if you want to grow your followers list, you should consider following writers who follow you.) But, do be gracious in retweeting information from other writers that may interest your followers. If you write romance, occasionally share information about a romance you just finished and loved. Do the occasional retweet of a promo if you have an interesting commentary on it. Which leads to the next tip...
3. Don't over-promote other writers.
There's nothing worse than following someone who does a rolling retweet of other authors' self-promos. In these instances, there's no thoughtful reason for sharing the information. It's like a mechanical thing of going through a list of promos and retweeting. (And yes, there are services that "contribute" tweets for you. I recommend avoiding those.) If you're somewhat selective in what you share, your retweets will mean more. Otherwise, your followers learn to ignore you.
4. Don't engage in too much self promotion.
Some people have likened repeated tweets about your own project to standing on a street corner and yelling into a megaphone. People walking by hear you (kind of), but there's a really low chance they're listening to what you have to say. Again, if you limit self promotion to two or three tweets per week at the very most, you're less likely to alienate followers. And make sure you have something new to say. Don't just tweet the same self promo every time.
Want to maximize exposure other writers are willing to give to your promotional tweets? Pin your best promo tweet to the top of your Twitter page. By doing that, you're letting new followers know what's most important to you. Many writers pin a tweet announcing they signed with an agent, got a book deal, or the link where people can buy their book. During pitch contests, pin your favorite pitch so others who want to retweet your pitch can find it easily.
5.Shorten links.
6. Interact with other writers in contests.
I found my amazing critique partner, Hayley Stone, and two other CPs through PitchWars. I tried to engage with other writers using the hashtag a few times a day. You don't even have to engage that often to connect with writers. Warning: once you begin tweeting on a writing contest hashtag, it's addictive because you'll find out how fun it is. Another warning: agents and contest judges are watching tweets on the hashtag. Don't be a d-bag.
7. Be careful with direct messages.
That means no automatic DMs. They're just bad form. Even a non-automatic direct message can seem annoying to another writer who just started following you. Unless you feel you simply have to send them a DM, consider tweeting at them and asking the best way to contact them. Then they can initiate a DM to share their email address.
What tips do you have for writers using Twitter?