Take a cue from university-level writing courses on this. Write a first version... Then walk away from it. Don't come back until you've thoroughly cleansed yourself of that particular train of thought. Return. Reread. Repair. Repeat until satisfied.

In a role play though you generally don't have that kind of luxury unless you have a lot of spare time on your hands. So I'd recommend just writing your first post, getting it out there, then going back a couple days later to read it over again and compare it to the other posts. Could you make it through your own post? No? Why not? Write down why not. Practice these things consciously in your next post. Reread, repair in future posts, recycle, repeat until satisfied. If there are methods of writing or certain phrases or words others use which you like a lot, maybe because it just sounds good or maybe because it's clever to you, reuse it. Repeat it. Learn where it fits and where it doesn't and keep doing it until it sticks in your mind as a subconsciously drawn from term. My favourite word of the moment is "ferreting", I'm using it everywhere.

The original Legend of Renalta, which lasted four years, went through [i]six or seven different versions[/i] before I finally found one that stuck through constant trial and error. Legend of Renalta 2, currently on this site, had its own stat system undergo four complete rewrites before I found one I was satisfied with. I wrote roughly six hundred traits and only used 1/6th of those in the final version.

tl;dr: You will write a lot of garbage before you write things you like, and even then, what you like, another might not. So the only yardstick to measure yourself by is your own expectations. Don't set it too far out of your own bounds but always seek to improve yourself through trial and error, learning from your mistakes... And don't be afraid to make them. You will never learn unless you try. If you never learn you will never improve. If you never improve, well... Then you'll just be stuck in that rut forever.