I gave a webinar on writing for the media last month where I tried to condense all of my years of education and hands-on experience into 60 minutes. That's no small feat. In the Q&A at the end—pretty much like every Q&A I have with every one of the continuing education classes I teach—I got an overwhelming number of questions about exceptions to the rules. This was my advice to them: Learn the mechanics of writing so you can learn when and how to break the rules to your advantage.
I don't care how long you have been writing or what grades you got in your high school advanced grammar class, your writing can always improve. And English is a living language, in part because of people who are brave enough to break the rules. (Thank you, writers like William Shakespeare, Lewis Carroll, J.R.R. Tolkien and Dr. Seuss for being brave enough to give us words like "swagger," "chortle," "tween" and "nerd.")
It doesn't matter how experienced you are, grammatical rules are constantly changing—may/can no longer have such distinct roles in our language, and words like nine-eleven and ginormous didn't exist ten years ago. Despite how much your grandmother might fight it, social media is changing language faster than ever: short-hand, comma usage, capitalization, word mash-ups. If you can't learn to evolve, words will leave you behind.
If you haven't taken an English class for 15 years, maybe it's time for you to enroll in a continuing education composition class. If you've never written historical fiction, take a library research or genealogy class. Or if you've only ever written science fiction, take an intro to journalism class. A history class will expose you to new ideas, a philosophy class will teach you how to support a claim or a literature class will help you understand good writing.
Each style of writing requires something different, and to learn those different elements, you have to be exposed to them and learn how to apply them. Even though I took more than a dozen composition classes in college, I still participate in writing conferences, lectures, courses and workshops. And every time I pick up a grammar book or style guide, I learn something new about usage.
I am far from the perfect example of a good grammarian (seriously, please don't read my twitter feed too closely), but a love for words and learning can compensate for a litany of mistakes.