Determine what are your interests.
By knowing what interests you, you can easily determine what are you going to post and through that, you can gain follower with the same interest as yours. Intereact with people with the same interests as yours.
By interacting with the people who has same interest as yours they might repin/rate your posts. Post eye-catching and interesting photos/videos.
People are visually-oriented. By Having eye-catching and interesting posts within your account you can easily attract followers. Connect account with your other social media accounts.
When your social media accounts are connected to each other your friends from your other account might seen you and follow you. Follow Back.
To gain more followers, you have to be friendly. Follow Back may also means 'I want to be friend with you too'. However, you are still free to choose who to follow back.