This is a warning, really, to not force trying to improve your writing vocabulary.
If one does, they are likely to not end up with the best words.
In fact, if you force it, you might even look a little stupid.
The best way to improve a writing vocabulary is the best way to improve all writing.
Read. Read everything. Read widely. And then write.
Write more. And then read more. Keep going.
And don’t ever stop.
Read every day and write every day, and you will improve in every part of your writing.
At least that’s what I try to do.
Put your vocabulary on the top shelf of your toolbox, and don’t make any conscious effort to improve it. (You’ll be doing that as you read, of course . . . . but that comes later.) One of the really bad things you can do to your writing is to dress up the vocabulary, looking for long words because you’re maybe a little bit ashamed of your short ones. This is like dressing up a household pet in evening clothes. The pet is embarrassed and the person who committed this act of premeditated cuteness should be even more embarrassed.
-Stephen King, On Writing