I agree with the others about being interesting. Stay away from gimmicks and you need to post often at least daily.
You can follow a lot of people each day, like not more then 500. Some of them will follow back. You can promote your twitter account on facebook, ask friends and family to follow you and things like that. And you can always buy twitter followers if you have a budget.There are many sites from where you can buy like SEOClerks, listingdock etc.
Through social exchange websites. These websites help you get traffic on different social media websites and even on your website.
Find similar twitter users and retweet their useful tweets. Hopefully they will check out your tweets and return the favor. The more retweets you get the more you'll be seen by other Twitter users. Another way of getting more Twitter followers is by adding a Twitter widget on your homepage. Not just a link or an image, but the widget where the user only needs to click on a follow link to instantly follow you.
Follow as many people as you can and in return they will follow you back. You can also buy twitter followers from sites like SEOClerks, many freelancers on this site provide twitter followers at a cheap rate.