One of the best strategies is to follow users in your niche and engage any that respond or thank you for following them.
Do not buy followers. Everyone was once a new account holder therefore people would not be afraid to be the first to follow you and you only need one follower to the first.
Follow users that have few followers themselves because they are more likely to engage with you and follow you back.
I am assuming this is from your experience since making the topic asking for advice? How well has your campaign gone so far?
One of the worse things you can do is buy followers.
Creating an effective campaign is about been genuine. Follow your niche, create tweets that are useful, if you have a blog it's an awesome tool to share information. Once you have a good following and you remain consistent with what is working, you will notice your followers increase on it's own.