How do you make it so it shows the whole picture and not just a square?
There are quite a few apps that will add the white borders to your photos, instasize, squareready, etc. I try to crop square whenever I can, as the accounts that feature photos typically will only share square cropped photos. It's kind of a pain, and some photos just can't be cropped 1x1 without ruining the composition, but the feature accounts always means more followers. It's really crazy to me how useful IG can be as a business, and building more followers is definitely a focus for me. More followers= brands trying to work with you based on your audience.
You'll see it blatantly being done by the "Instagram models" (hot girls with huge followings, some over a million) where they are promoting so-and-so shoes or clothing, but it's not very subtle and really just kind of corny to me. What I try to do is promote the companies I work with in a more organic way, plus I only work with brands who make products I actually like and use. Granted, I know my reasons for using IG are far different than most photographers who just want to share photos, but I like to show that Instagram is worth putting effort into as a business.