The best way to improve writing will vary largely by person. Here's what worked/is working for me that I think generalizes better than average:

1. Get good enough that, when you read other people's writing, you notice things they could've done better. Notice the first step in "how to improve at writing" is "improve at writing"; going from below baseline takes a different strategy than going from baseline to above. Importantly, until you get to the point where you're able to make improvements to other's writing, writing your own stuff isn't going to help, since you're not high level enough to improve it. That is, getting to this baseline is going to require external resources. I personally used a combination of Strunk and White and a MLP fanfic blog.

Also, getting to this level doesn't need to take much. You can get absurd mileage out of "Omit needless words".

1. Find a person other than yourself who's writing something and would like a beta. Things that you should probably look for: intelligence, similarity in writing goals, divergence in writing style.

2. Act as a beta reader. Make suggestions liberally. That is, if you see a change you might make and are unsure if it will improve the writing, suggest it; your primary will either accept or reject it. The important part is you discussion points of disagreement.

You can, of course, go at this from the opposite side (as a primary who finds a beta), but my system 1 says the other way is easier. They guy I currently beta for had to put in an absolutely absurd amount of work to get to the point of taking on betas, whereas I just had to leave a review saying "this is pretty good, but you could improve x, y, and z, and you could really use a beta, and I'd be up for it because this story will be awesome if you can clean this stuff up."