#4. Use third party tools such as crowdfire, statusbrew to see 
, your inactive followers etc so that you can unfollow them.
#6. And most importantly, be active by liking and commenting on photos.
Now that we're done with the obvious tips. Make sure you've followed them before continuing. 
Have A Theme
In order to have a really successful Instagram account, you must stick to a theme/niche that interests your followers and develop a fan base based on that theme. E.g: In case you are a designer, then you can post about the fashion industry. If you are a chef, you can post about food. etc. Make sure you only post unrelated content to your niche/theme 
occasionally. If you do it frequently, you might end up losing followers.
Have A Schedule
In case you have a schedule, your followers will know when to expect a new picture and they will wait eagerly for it. Have a rhythm and maintain it. This does not mean that you must post daily, although it is recommended to do so. You need to have a specific schedule and stick to it in order to have a loyal following. Also try to analyze the time zones of the majority of your followers so you don't mistakenly upload photos while they're sleeping. Remember, More likes = More exposure = More followers.
Do Not Bore People
People have an expectation from you. In case you have been posting photos of your dog and kitten, then do not throw in a picture of morning sunshine. Give people what they want. Hence try to have a variation in your photos but do not move away from the theme. Look at your own photo and see if it bores you. Now decide what you want to post. Always have something new and different to share. E.g: If you're a fashion designer, you can post motivation quotes or photos about New Technology.
Have Interesting hashtags
It is a good practise to use hashtags on your photos. Hashtags will help you to get more eyeballs on your post. Hence do not stick to common hashtags. Instead, you must stick with a handful of unique hashtags that are related to the theme of your account. It's best to research hashtags to see whether it suits your photo or not, before using them and make sure not to overuse hashtags.
Set profile to Public
Most people expect to become famous overtime while having their account set to private. By doing this, hashtags will be useless as your photos won't appear when people search those hashtags. If you plan on gaining a lot of followers, make sure to set your profile to public.
Interact With Your Followers
This is your way to acknowledge your followers in some way and show proof that you are alive, not dead. You need to let them know that you appreciate them by liking their photos, commenting and also tagging them when possible.
Post Good Photos/Videos
This is a obvious one but a tip that most articles fail to mention. By posting good photos and videos, you will obviously gain more likes, resulting in more followers. Post photos/videos that you know for sure will interest your followers. Take photos that can be as good as possible and make sure not to just post photos all the time, but also videos. Simply master a few photo basics in order to take a good photo.
Edit Properly
This means that you must invest in a good photo editing app. Hence you need to go much beyond the limited filters as well as editing tools of Instagram. There many 
photo editing apps for your smartphone which can help you achieve this.
Get Featured
Your Instagram account cannot exist in a vacuum. Hence try to develop relations with famous Instagrammers and have them feature your Instagram account by giving you a shoutout on their profile or you can blog/guest blog and link your Instagram account to direct readers to.
Take Good Selfies
Try to take good selfies. This may require you to undertake a short online course to learn basic tips such as: Make sure of simple things like keeping the focus on your face. The light should be coming from infront of you or else it will make your selfies look dark. Wear light colored clothing so that the image comes out well. Have a steady hand or else try to make use of a selfies stick in order to have impressive selfies or take photos using 
selfie apps, that contain filters which will make you look good and save your time from editing.
If you follow these tips correctly, you'll be able to get more followers on Instagram and most importantly, also make sure to maintain them by staying active. In simple terms, the more followers you have, the more followers you will get!