I don't think there is a single best way here. You'll need to figure out what works for your audience.
If you're just getting started, I'd focus on two main things early on: quality photos, and consistency. Make sure photos are taken in a well lit place (preferably outdoors, if you don't have a studio). Take photos of apparel on models, not just on the rack. And post something fresh at least daily, and perhaps more frequently if it seems appropriate.
For audience growth and engagement, you could try Instagram contests. I've several different versions of contests come across my feed. Requesting instagram users to comment, repost, mention friends, etc seem to be somewhat common. Just don't get too annoying with it.
Alternatively, you could ask for user feedback. For example, post a split image and ask IG users whether they like the left image or the right image.
I also like the idea of growing audiences through website engagement. For example, in your ecommerce store, you could offer an immediate discount code if the user follows you on Instagram by using a website plugin like JustUno.
I'm happy to discuss more ideas about how to leverage IG and how it can integrate into your digital marketing plan - just request a call.
I also have two friends who are into women's fashion and have a successful following on IG. One is a retailer and one is a blogger. I can share some of their successes with you if you want to connect.