I can’t really offer a guide on how to gain Twitter followers because:
1) Who am I?
2) Everyone uses Twitter differently, and I won’t assume you want to use it identically to the way I do.

Knowing how you want to use social media is probably a good first step in gaining followers. But other than that, I have no idea how to make people follow you. (Don’t be boring? Give them cookies?) What I can provide, however, is a list of things I see people do on Twitter that make me want to unfollow them, or even block them.

Tweeting too much.
If you have less than 1,000 followers but have tweeted over 80,000 times, I am suspicious of you. It’s not that what you’re saying is “bad” necessarily, but it means one of two things:
1) What you are saying is not effectively building an audience.
2) You have no interest in building an audience, and are treating Twitter as a sounding board for whatever pops into your brain.

Not having a Twitter avatar/not tweeting enough.
People like following humans on Twitter! Make friends by eliminating the Lifeless Twitter Egg and tweet at least once once a week. Also make sure your tweets say more than “I don’t know how to use Twitter” or “I don’t tweet enough.” Because… why are you even there? No one forced you to join. No one should join a social media site if they have no intention of using it.

Only promoting your book.
I’m cheating with this one a bit because I, personally, instantly mark these people as spam. So, I never really see this on my feed. If a writer pitches me their book via Twitter (when it’s not for a pitch contest) or sends me a link to their Amazon page, I click on their profile and 99% of the time, they’ve sent almost every other agent on Twitter that exact same message. THIS IS NOT MARKETING. It’s spam. Similarly, writers who don’t have books of their own will spam Twitter feeds another way – by only talking about their friends’ books. Like all book promotion, this, too, should be limited.

Not having a clue what you’re talking about.
This is obvious, but sometimes I see industry folk (usually newbies who are eager to impress) and authors discuss “publishing” and realize they don’t actually understand the business. I’ve been an agent for 3 years and worked in the industry for a little over 6 years. I’m no longer a “newbie” but I certainly have a lot left to learn. I’d like to believe I’m smart and capable of being the “rock star” I’m sometimes referred to online, but I’m not… yet. So, when I don’t understand something, I do not tweet about it. And if I do this by accident, and am called on my bullshit, I curl up into a ball and die; I do not keep tweeting and being all indignant about my lack-of-knowledge.

Streams of consciousness that turn into floods. 
Similar to tweeting too much, tweeting too often and not staying consistent is also something that makes me click Unfollow. For example, using Twitter to talk to about an article you read, and quickly realizing it’s going to take about 5 or 6 tweets in a row to get your point across. THIS IS NOT A BAD THING. Nor is taking time to respond to others who add to the conversation. What does look unprofessional is when someone turns my feed into this:

“This topic merits discussion. Here’s a link _______” [2:22pm]
“Here is Opinion #1 why this topic matters” [2:24pm]
“And Opinion #2” [2:25pm]
“@follower1 I agree because of reasons!” [2:28pm]
“I have to bring my cats to the vet today. Sooooo sad.” [2:29pm]
“@follower2 Oh, I disagree. Did you not see that link?” [2:30pm]
“My book has a pub date!!!! Please pre-order it from Amazon!” [2:31pm]
::SEVERAL RETWEETS ON VARIOUS TOPICS IN A ROW::
“I’m so excited for fall to start. Pumpkin spice latte season, y’all!” [2:32pm]
“@follower2 Let’s keep arguing about that other thing for a while. Does anyone even remember what I posted before?” [2:33pm]
“Here’s an adorable kitten GIF. Because Mondays, right?” [2:35pm]
“Did you all watch Breaking Bad last night? OMG!” [2:36pm]

I see this often, not just from writers, but from agents and other industry folk. There’s nothing wrong with any of the above-mentioned tweets individually, but spewed out in a 15-minute chunk is a problem. Yes, you should have variety in what you tweet about and engage in conversation and let your followers know about any new developments with your book. But, basically, chill out. Space out your tweets and understand that not every thought you have needs to be shared. The information you really, really want your followers to know will end up getting buried. Usually when I scroll through my feed, and I see the same person appear 10 times in a row, I’ll just read whatever their most recent tweet was. Because… ain’t no one got time for that.

Too. Much. Information.
Revealing how much of your personal life you share online is, of course, up to you. If you’re using Twitter as a tool in your professional life, however, be smart about what you say. Do we need a live-tweet of the birth of your child or a Vine of your colonoscopy? NOPE. If we learn your children’s names, do we necessarily need to know them as well as our own family?

Granted, I’m skeptical of how “social” every single website has become, and given the actual physical attacks on literary agents that have made the news, I’m about one step away from living in a bunker. I’m also just as much of an introvert online as I am in person, and for me that means needing my privacy. I understand not everyone is like that. But, if you do share, remember not to over-share and make things uncomfortable. Don’t be that guy at the party who brings a pleasant conversation about books to screeching halt because all he wants to do is publish a book to prove he is good enough, mom, and why doesn’t anyone love him, I mean, really. Twitter shouldn’t count as your weekly trip to church, therapy, or be a substitute for coffee with your real-life best friend.

I’ve said before (on Twitter) that social media is like a cocktail party – fun and casual and not as buttoned-up as the office – but some networking and shop talk will occur, so don’t get too drunk. Any actual business (e.g., pitching your book, information about submissions, following up on queries, etc.) should be saved for the office, aka “email.” Now, go forth and make smart social media choices!