Simply, read more.
Okay...not *simply* read more, but read more WELL WRITTEN books by great authors.
What? You say you don't have time to read more because:
1. You're spending all of your time writing.
2. You have a full time job, and you're trying to write.
3. You have thirteen children, a full time job, and you're trying to write.
You still want to become a better writer?
It's still the same answer. Oh, and:
1. Write more
2. Get a critique partner (or four)
3. Read excellent books on the craft of writing
4. And, read more.
What's the best tip (aka secret) you have for becoming a better writer?