I wanted to add on to yesterday's post. The reason that my vocabulary is so strong is that every word I know is connected with several others by numerous principles. Every time I learn a new word, it connects with other words I know, as well as other linguistic principles I have.
For example, I mentioned the word "bespeak" yesterday. When I heard it, the first thing I thought about was the word "belie". Well, guess what. "Bespeak" and "belie" are basically antonyms. And since "belie" means "to show something to be false" and has "lie" in it, then I know that "bespeak" must be the opposite, meaning "to show something that is true."
Any time you learn a new word, understand that it does not exist in a vacuum. It evolved from previous words. It belongs to the same family as other words - words you probably already know. This is another great reason to learn grammar and other nitty gritty aspects of English (Latin and German don't hurt, either). If you can analyze and break down a word into its smaller components, if you can track down the history and evolution of the word, you will have a deeper and stronger understanding of it, even if it is the first time you ever saw that word, and you will be much more likely to retain it and use it in the future.