All writing will leave an impression on the reader. That impression may be good; they find your writing to be witty, well-composed, and thoughtful. Or it may be leave them wishing they never began. Their impressions of your writing may flow out of your writing style, but even the smallest error in grammar can be a blinding distraction to devouring and enjoying your work. While learning your writing style takes practice and time, we can all stand to make more of an effort to avoid common grammar mistakes.
This is an area I am still growing in as a writer. It is by far one of my weakest areas. So rather than pretending I'm an expert in common grammar mistakes, here are a few actual experts and their sites I've frequented often for their excellent grammatical advice.
Inc.com (a resourceful website for more than grammar)
15 Grammatical Errors that Make You Look Silly (great infographic for quick grammar review!)
Each of these resources are great for a quick glance and review of your work, but if you're looking to take your knowledge of grammar one step further consider purchasing a grammar book. I still utilize my writer's companion from college writing and I'm looking to update my edition as it's a frequently used resource in my writing.
What grammar resources have you found helpful?