There is no better way than reading and looking up words. Poe is a great one for adding to your vocabulary. Trying to use a word in a sentence is an excellent practice. And you can check your sentences here with us if you like. (I’d suggest gathering up several at a time and putting them into one question.)
People say that I have an excellent vocabulary and command of the language, and in general I think that’s true. But this doesn’t mean I don’t have to look up words. I look up words all the time. I have five big dictionaries close by (I love dictionaries), and for quick reference I go to Dictionary.com. I check many words before I use them, just to make sure I’m clear about their precise meaning. And I like reading things written by people who know more words than I do (Michael Chabon comes to mind) because that stretches my vocabulary.
Another good way is to study another language, one that has a lot in common with English. Four great choices would be Latin, Greek, French, and German. You don’t have to know their grammar to get a benefit in vocabulary. Just studying a list of words and what they mean, with special attention to the roots, will help you with English.
One approach to roots is to look for words that have a core element in common; for example, words with port: porter, portal, transportation, import, export, portable, and so on. When you know that the port root is Latin for “to carry,” you can see a relationship among these words that illuminates their meaning. Not everything that looks related is really related, but just going by what you can see will teach you a lot. Notice that this is one reason why spelling is important. (Important: carrying weight, in the sense of having consequence or significance.)
You can also learn what the common prefixes and suffixes of English words mean (for example, per-, pro-, ex-, ad-, -fy, -logy, -phile, etc.). Many of them do come from Latin and Greek. If you know a lot of roots and a list of prefixes and suffixes, you can figure out a great many words that you haven’t learned directly.