There are some great apps that are free to add followers such as http://followtorrent.com/ and http://www.followback.club They are worth up to 500 followers a month. The people with the largest followings on twitter can use other tricks and still not pay a penny. Remember alway follow for follow. If you add people always check they are following more people than they follow - no one needs vanity friends on twitter. On problem may be you get suspended because you reach your following limit. To avoid this keep those who do not follow back under control use https://unfollowers.com/ Best of luck and may you mutually follow tens of thousands of followers.
The strangest thing has happened. Followback club and Follow Torrent have just been closed down by Twitter. It shows the success theses apps were to add followers. At least I never paid a subscription.