A great way and fast way of building Instagram Followers is to promote your dedicated hashtag on your other social profiles, on your website, and your email blast.
If you have a Youtube channel you can also use your Youtube Channel to promote your Instagram Account and increase your Instagram Followers.
Follow other Instagram Accounts as well which helps you gain Instagram Followers because other Instagram users will see and notice your Instagram account and if they are interested they will Join.
I increased my Instagram Followers by promoting my Instagram Account on Youtube, On my Blog and through Forum signatures as well.
Using Twitter to promote my Instagram account also helped greatly.