Be prepared to put in hundreds of hours of practice. A common misconception is that pixel art is "easy" but it's like any other art discipline. It's a long and sometimes frustrating road, but ultimately satisfying. Even fairly simple 16x16 sprites have a real art to them to make them look remotely good. I've found the following has helped me in my quest to become a better pixel artist. I still have a long way to go.
1. Collect a whole range of pixel art that you can look at for techniques/inspiration. Services like Pinterest are great for this as you can easily tag and catalogue any cool art you find and then you have a library to draw from.
2. Try to do a little bit each day, even if you don't feel like it. Join something like #pixeldailies where they set you a task that you have to pixel each day. It's a great way to build up practice and also a library of pixel objects.
3. Look at a lot of old games from the 8 and 16 bit era, download spritesheets of these and see how the animators set up the key frames, colour schemes etc. You can glean a lot of knowledge this way.
4. Use pixel art in instances where you normally might not. Got to do a birthday card for someone? Do it in pixel art. Need to make an animation? Pixel art. Want to paint landscapes? Pixel art.
5. Get feedback. The pixel art community are oddly fanatical, they can get hung up on traditional techniques/limitations etc and it can be frustrating. That said though, if you want to have your work dissected and brutally critiqued you've come to the right place. As hard as it is, it's a good way to get better.
6. Practice with purpose. Don't just draw heaps of things and hope to get better. Try different techniques otherwise you will hit a plateau and not improve over a certain point.
7. This one might be a bit weird, but I found that knowing how much time I've spent doing pixel art to be motivating. I do some of my pixel art in aseprite through Steam which counts the time I've been working. I find this oddly motivating seeing that I'm working more on pixel art than playing games. your mileage may vary.
Hope that helps a bit. It's a long road so just chip away at it slowly. There's no magic bullet to get better over night.