To have a successful Instagram, I think one of the most important things is to establish your aesthetic. I am not claiming to be an Instagram pro at all, but here are some things that have worked for me and that I have noticed working for other people!
1. Add variety.
Have a variety of shots. Far away, close up, flat lays, detail shots, etc. Overall this is going to make your feed flow better and have a general theme that has variety. Content shots are also something I started implementing to my feed with in the past year and I think make a theme a lot stronger. By content shot I mean, a pretty drink, sunset, beach, food, etc. These photos generally don't get as much engagement, buuuuut make your feed stronger in my opinion so I think they are worth it.
2. Edit consistently.
I personally edit with Lightroom presets for consistency, but you don't have to spend money on presets to have a consistent feed. There are so many apps out there that will give you the same look for all your photos. I used VSCO for YEARS before I started using Lightroom. I had one filter I used and tweaked for all my photos. Other apps I love are Lightroom mobile, Snapseed, and Mextures.
3. Plan your feed.
Planning your feed is a game changer. I use UNUM to plan my photos and it is the best app I have ever used to enhance my Instagram. By planning out your feed you can see how everything will look together before you post the photos.
4. Don't be afraid to try something different!
Sometimes you will 'break' your theme and that's okay! You have to experiment and try new things. Don't be afraid to completely change your theme!