Here is a quick white hat guide to getting more IG followers:
Upload legitimate content even if you didn't take the photo. Exotic cars, funny memes, beautiful celebrities, popular tweegrams. Content that people will generally like regardless of the interests they identify themselves with.
Use as many POPULAR hash tags as possible on your pics. The current limit on hashtags per picture is 30(last time I checked).
-An example hashtag list for followers would be: #follow #followme #followforfollow #f4f #teamfollowback #pleasefollow #followme (you get the idea? google search the most used hashtags that you think are relevant to what you're trying to accomplish). I go for Likes, not zombie followers, so I use popular tags such as #likeforlike #iphoneography #bored #happy #ignation #igaddict #iphoneasia...
One way to gain more followers at random is going to the most popular photos and like them. At this point it sort of puts your username on a "list". People use photos with thousands of likes for a easy "list" of people to follow(why auto follow people on multiple photos that only have 30 something Likes when you go to to one spot/photo that has 2000 Likes/people to follow). The results are mediocre but you might gain 3 or 4 peolpe per day if you like 20-40 popular photos.
Of course you can always just spam follow as many users as possible and a lot of times they will follow you back as long as your pictures don't completely suck. I think most people assume that you're generally interested in their pictures when you follow them and will generally follow you back out of respect/thanks.