Twitter have bug, which you can use to increase the number of followers you have. This bug allows you to increase the followers count when ever you are hitting the follow button from another account multiple times.
What you have to do is
Login to another twitter account
Open the twitter handle (where you need to increase the follower count)
Just click the follow button multiple times (follow/Unfollow)
The count will increase
See the images below
before
After
If you don’t want to click manually you can use JavaScript which is described here, This is the blog of Karnesh Mehra who reported this bug.
The followers count will be reset to the correct count after few hours. Also when you are unfollowing it is showing the exact followers count.
You can use this just to increase the followers count for some hours only.