Hey!! What's the story?
One method I recommend Jamie & what I tell aspiring writers who ask me, is to read, read and read again. Immerse yourself in different genres & styles, worship variety & develop a love & passion for language & keep that literary fire burning!
The best to develop vocabulary is to just read a lot and look up words that you aren't familiar with, then write them down, etc. However, like Stephen King says, ANY word you have to look up in a thesaurus while writing is the wrong word. Never underestimate the power of simple language. Monosyllabic words are more powerful than polysyllabic. As Winston Churchill, one of the all time greatest orators of the the English language, said, "...short words are the best, and the old words best of all.' Think of phrases that stick, that resonate throughout the years, decades, and centuries:
"To be or not to be." (Six monosyllabic words. Not, "Whether to partake of this corporeal existence or amalgamate with yawning oblivion")
"I have a dream." (Four monosyllabic words. Not, "My possession is an aspiration.")
"It was the best of times, it was the worst of times..." (12 monosyllabic words)
"Ask not what your country can do for you..." (only one word with more than one syllable and it has two)
"The only thing we have to fear is fear itself." (same as above)
"Friends, Romans, countrymen, lend me your ears...."
You probably got the point several quotes ago. But to put a fine point on it: short, common words are almost always the best choice. Thesaurus words can help with characterization in dialogue, but reading for vocabulary is always better. If you want a character to sound like she is a 16th C. countess, read literature from the period and study the history.
My parents were both English teachers. The standard answer around my house if I asked what a word meant was, "You know where to find the dictionary." I still use both the dictionary and the thesaurus regularly.