I will usually post every other day. It is usually a finished piece but if I don't have one yet I will post a detail of a work in progress. Just be creative, post an artistic picture of your tools, your motivation, your furry little helper (if you have a fur baby) or a selfie with a fresh canvas talking about what you are going to do.
Also, work on staging. Hang your painting on a beautiful wall to show what it will look like in someones home.
Lastly, do not follow someone just to get a follow. (Not that you would, just a tip for all) This puts me off from someone on instagram. And don't unfollow after. I will take some time every few days and look through new art on instagram and support those artists. If yoi are regularly communicating with other artists, liking their work, they will most likely remember you and do the same for you. So, even if you are not posting anything that day, get on there and search out new artists, they will most likely check your page out as well.
I did all this and got over 300 followers in a month. Good luck, be yourself and share the love!