I second what everybody else says; read, read and read again, and in particular read works which do not come from the milieu that you are already familiar with. Read classic novels. Read novels from other English-speaking cultures. And, of course, read poetry.
Reading definitions of words, outside the context of regular use, will only get you so far. You need to encounter words being used, and repeatedly used. If you can, listen to radio broadcasts from other English-speaking countries. (And you can. That’s what the Internet is for.)
And, if you’re really committed to this, move to Ireland. I’ve read - sorry, no cite - that, of all the Anglophone communities, Irish speakers of English have the largest vocabulary of words in daily/regular use .
If you have iron discipline, get a guide for the SAT verbal section: it will have a list of vocabulary words.
But that’s a drag. An entertaining way to improve your vocabulary by a small amount is to borrow or purchase Word Power Made Easy. The author provides both memorable discussions and exercises; he has a nice sense of balance.
Read all sorts of stuff. Branch out, read modern stuff, read stuff from other English speaking countries, read Victorian works, read Elizabethan/Jacobean stuff like Shakespeare and the King James Bible, prose, poetry, everything.
Victorian poetry seems quite good for vocabulary building. I personally like Tennyson.
Another tip: if reading definitions doesn’t “stick,” then read some books on word origins (etymology). Having the story–or stories–behind the word can help plant it firmly in your brain.