I know that I've tried very hard to make my writing good, often to the point where it becomes bad in an effort to avoid silly mistakes.
For example, I've been trying to avoid the word "that" in my writing, as that's supposed to be a weak word. However, in my efforts to cut out the word, I often end up with grammatically ponderous sentences--sentences which could easily be shortened and streamlined just by adding the word "that" in the right place.
I've read books where the author has tried just a bit too hard to be funny. To some authors--like Terry Pratchett or Glen Cooke--humor comes naturally. For other authors, the humor comes across as almost a bit forced. The "humorous" dialogue is either unrealistic, choppy, or the jokes are a bit too "clever" that you just don't understand what the speaker is hinting at or alluding to.
Other writers make the mistake of trying to be too eloquent or fancy in their writing. If you don't have both a dictionary AND a thesaurus handy, you will never understand what they're trying to say. I've seen the eloquent writing of masters like Dickens or Bronte, and then I look at the simplicity of my own writing. There are maybe 10 words in there that readers would need to look up in a thesaurus or dictionary, but other than that, it's just basic English. Instead of trying to force eloquence--which would probably make my writing come off boring and incomprehensible--I keep it simple.
Here's a free piece of advice: don't impress the readers with your WRITING, but with the story itself.
You don't want readers to look at what you wrote and think about the quality of the writing, as that will distract them from what you want to say. In fact, you want the writing to fade into the background, and simply have it as the vehicle to tell the story you want to tell.
The whole point of writing is to communicate, and you don't want readers to get so hooked on the method of communication that they fail to receive the message. It's like calling from a broken telephone with a shaky signal--the person on the other end will know you're trying to communicate, they just won't understand what you're trying to say.
Write in your own way, but don't try too hard to make your writing something it's not. Play to your strengths, and let your writing paint the picture that you want it to. The words should fade into the background as your reader builds that mental image in his or her head.