Don't do it.
Tens of thousands of followers might look good on your home page, but if those followers aren't reading your tweets, looking at your blog posts, and buying your books, they are just meaningless numbers. What you want is real followers, people who are interested in what you have to say, and in what you have written.
How do you get genuine followers?
First you have to find your ideal audience. There are several ways to go about this using three essential tools: Followers, Twitter Lists, and Hashtags.
1) Make a list of successful authors who are similar to you. Look at their "followers" list, and follow accounts that are active. (It helps if they have a significant number of followers - more than 2 digits. Avoid accounts that look like spam, or that don't appear relevant.) This may take a little time, because you will have to actually look at their accounts and see when they last tweeted. (Ideally they should be tweeting at least five times a week.) There is no point following people who don't tweet, because they will not tweet about you. It seems selfish, but the best use of twitter is not what you tweet, but how many people are willing to re-tweet. That's called marketing.
2) Reviewers are your audience, too. To find book reviewers, do a search on #review plus your genre. Ex. "fantasy review." This will produce a list of recent reviews. Click on the account and if the reviewer has tweeted consistently about reviews/books, follow and add them to your list of reviewers. (Note: If you search "reviewer" instead of "review" a list of promotion companies will pop up.) You can also look at the followers on popular review sites.
3) Don't follow blind. Before you follow people, read their recent tweets. If you are interested in their tweets, chances are they will be interested in yours.
4) Look at the Twitter lists of authors in your genre, as well as businesses that promote books, publicists, agents who represent your genre. Do they keep a list of publicists, promotion sites, reviewers? If an author who writes in your genre keeps a list of reviewers, odds are they will be interested in your book. (Make sure you mention how you found them in your query, or in your tweet to them.) If the list is public, and looks as if it will be useful, you can subscribe. Then do steps 1 and 2 above.
5) Expand your scope beyond writers. For example, if you write about politics - political thrillers included - find people who have similar political views to yours. (Use hashtags to find them. Ex #progressive.) Chances are those people will be interested in what you are writing simply because you share the same point of view. If you write children's books, tweet about parenting, education, and other topics that interest people raising kids. Be engaged with the world.
6) Express yourself. If you feel strongly about something, don't be afraid to have an opinion. You want followers who believe in what you are willing to stand up for. Writers are leaders.
7) Tweet at least 5 times a day on different topics. Vary your tweets to include some with images, some that are an image only, some that are a comment, some that include a link (make sure to shorten it with bitly), and some personal news (your upcoming release, a new project you are working on, etc.) Check to see which tweets get the most responses on Twitter analytics. The tweets that are the most popular are an indication of what your audience likes to hear.
8) Use hashtags in your tweets. People who are on the lookout for topics, genres, free books on Kindle, and news events will search for them using hashtags. (If you aren't using hashtags, your tweet will get lost.) You can use those hashtags to find people who tweet on specific topics. Check out their home page, and if you like what they tweet, follow.
9) Don't be afraid to use Twitter to communicate directly with people - even if they are not your followers. I find that people who tweet me get my attention. Whether they are commenting on one of my tweets or offering something, I almost always respond, either by tweeting back, or by checking out the link they've sent me. Literary agents are more likely to respond to a tweet than to a query. To get the attention of readers and reviewers there is no better tool than direct communication via Twitter. (Don't DM - direct message. People resent DMs. Just mention them via their handle @personyouaretweetingto.)
10) Make sure your bio includes the information that is relevant to potential followers. For example, if you write sci-fi, include your genre so that sci-fi readers can find you. Don't include the fact that you have three kids, unless you specifically want Moms to follow you. Include what you tweet about and a personal closer that will get attention (e.g. something witty, funny, cute, dark - anything with a punch). Ex: "Author of scifi novels & short stories. Tweets about astronomy & Indie publishing. Building a time machine in my basement on weekends." Always include where you live on your profile, your website, and a head shot. (No eggs!)
Finding your ideal audience will take time and patience, but if you devote 15 minutes a day you can build a significant following within a few weeks. Two thousand active and engaged followers are worth more than 10,000 followers who exist in name only.
Related posts:
225 Hashtags for Writers
Platform, Shmatform: Social Media - How Numbers Lie
Twitter: How to Build a Following - for Writers