Get connected to your other social channels. Not only will you get more followers but any image you share on Instagram will automatically update your other profiles.
Respond to comments - followers love it if you respond directly to them. Make a point to check and reply. Try to connect with these people on other sites.
A great way to encourage followers is to invite them to upload their own pictures to your stream. Turn it into a competition.
Here's a fact. Images on Instagram that had a dominant colour of blue got 24% more likes than other colours. Blue is the attention seeking colour.
Be consistent - keep your posts flowing and your audiences attention.