This one is too easy, but it's so important I had to post it:
1. If you don't have a dictionary, buy one.
2. If you don't use a dictionary, start.
3. If you haven't explored your dictionary, do so.
Part and parcel of the same topic:
A. Read your own work. How many times do you use the same verb, adverb, modifier to describe and detail something in your story? Learn what your "catch" words are and find alternatives. Yes, this is more in line with Thesaurus use, but learning NEW words and how to use them will make you a better writer.
B. What are your "problem" words? I know that when the story hits me strong and I'm moved by the muse, the words fly from my fingers too quickly to catch the correct use. My problem words are "there/they're/their;" "to/too," "staid/stayed." I have others, but this is the short list. I keep them posted above my desk, adding to the list as I discover additional words I frequently misuse or misspell. Awareness of those words helps me make fewer errors when I am writing and makes me a better editor as well.
Our language is so complex, dynamic and beautiful in it's variety, it's a shame to limit ourselves to the same stale expressions over and over. Try something new. You just might like it!