1. Spend all day following other accounts, some of them follow you back 2. Tweet extremely contraversial (but legal) political stuff 3. tweet loads of random keywords - there are plenty of bots that will retweet or follow based on a keyword list.
... but i'd suggest you work out why you want to increase your followers, that would give you a better idea of the kind of followers you want and you could then plan how to get those type of people to follow you.
You can try humor (but pick and chose topics carefully, won't know who might be offended). You could be an activist (but be careful, you might alienate whole swathes of the web). You could provide insightful information (but might have folks disagree). Run for political office (sure to generate a lot of tweets, but then having to actually fund a political campaign might be expensive).
There's no quick way to increase followers. The paid versions have been caught out and are being shut down. Fake and bots are pretty useless.
I'd go with QuarterPan's suggestion: Be Interesting. :)