When I wanted to improve my grammar/writing skills in French, I hired a tutor (he was a friend of a friend – a French exchange student). Any native speaker could do this kind of tutoring:

Each week, I would write 2 or three pages in French (on whatever topics I wanted – usually things that interested me, for that’s what I’d like to be able to say properly in French), and he would come to my house, read it quickly, and then spend about an hour correcting it with me.

Because I was there, if I wrote something strangely, he could ask me if I meant to say A or B… We did 99% of the corrections in French, because I am okay at conversation/ comprehension, but my biggest weakness is grammar.

And then I also had a written record of all the mistakes I typically made in French. If we had extra time left, we’d usually chat for a few extra minutes about the topic.

I found it really useful for my level and my specific weaknesses.

By the way, 43Things in English is a very good way to practice writing!