A large vocabulary is like a box in the attic. It's like one of those excitingly dusty large boxes in the corner, one that you can pull out and dig through and find treasures you barely knew you had but turn out to be just the thing when you need them. One of those boxes.
You don't get boxes like that for nothing. Boxes like that take a lifetime of work, hoarding, finding, secreting, shuffling, treasuring, discarding and rediscovering. Increasing your vocabulary is like that, too. You're stocking your attic box so that when you need to appear somewhere in something magnificent, unique and unexpected, you've got just the thing. This, then, is a small guide to increasing your vocabulary.
DISCOVER YOUR PURPOSE
Why you -- you specifically -- want a larger vocabulary becomes your first decision. It may be that you've got a huge standardized test coming up, one with a vocabulary element, that determines the outcome of the rest of your life. Perhaps you're trying to get a second or third or twelfth language under your belt. Possibly you're entering a new career field and you need the jargon. So, determine your purpose.
Ultimately, the purpose of having a large vocabulary is flexibility of expression. You can say precisely what you mean and create a strong understanding. Standardized tests are a (poor) way of finding applicants who will have this ability. In any language, finding yourself stuck without a word for the thing you want to say gets between you and the other speakers of that language. Entering a new career without the jargon leaves you looking like a hack.
So, now you've found your purpose and you're ready to begin.
FIND YOUR WORDS
Now you've got to find a database. Somewhere, you've got to find the words you want. Depending upon your purpose, the lists will crop up in different places. There are vocabulary books for every standardized test, and for every language, and trade publications for every career. (Even Transvestite Electrical Engineers).
Remember that language has four aspects, and you must increase your facility with the vocabulary in all four of these aspects:
Find your words in both forms, that is, a listening source and a reading source. Obviously, a standarized test wouldn't have a listening component, but in the grand tradition of killing two birds with one stone, you might want to permanently add those words to your life by adding the listening element.
Having found your database, you will now want to set about filing those words away for use when you need them.
COMMITTING TO MEMORY, ENABLING RECALL
One crucial fact of memory is that if a thing makes sense to you, you will remember it. Conversely, if that thing does not make sense, you will not remember it. Therefore, find a way to make sense of each element of your list. Another important thing to remember is that your mind files memories by attaching new ideas to already-stored ideas. So, find something you already know that relates to each element of your list.
As a personal example, I have found learning Spanish vocabulary easier if I can relate it to the Latin I picked up so easily as a middle-schooler. If I think of Spanish as Latin learned by overhearing conversations through thick doorways, it all makes much more sense. So to me, the fact that "they are" in Spanish is "son" makes sense to me, since in Latin it's "sunt". See? Like you heard it through four inches of wooden door.
Having said that, there are a few tips and tricks that almost always work.
TIPS AND TRICKS
First of all, you should be able to recognize each word as a separate entity. Lists are ok, but please remember that you will remember the first 40% of the list like your own birthday, the middle 40% of the list will not find a place, and the last 20% will be iffy. Break up that list. Put each word on its own index card. Go through the rotation. Separate the cards into piles of "Words You Know", "Words you Sort of Know" and "Words you Do Not Know". Go through the second of those piles twice and the last pile three times. Then shuffle the index cards and put them away.
Make time each day for your index cards. A great way to do this is to keep your stash by your bed. In the morning, wake up and look through your cards. When you go to bed at night, go through your cards. It's easier to find the time to review your words if you use those groggy minutes. You're vulnerable at those moments anyway.
Create associations in your memory with these new words. This facilitates recall. If you have several associations with each word, you can recall it when you want it, rather than five hours later when you don't need it. This is where the mnemonics come in. Roots of words can be mnemonics if you know the language the root comes from. I had one mnemonic, not a root one, for the word "deter". To me, it kind of looks like "Derek Jeter" who is a "shortstop." So it kind of makes sense that deter means to prevent progress. I remember that both your fingers and your toes in Spanish are "sus dedos" because my husband calls me "Mono" and monkeys have hands and feet that can manipulate. And there are several others.
Read more material with words you won't recognize. You are reading at your level of vocabulary competence if you don't know only two or three words on the page. Once you don't recognize five to ten words, you have something at the next level, and this becomes an appropriate self-teaching tool. Make cards for those words and add them to the rotation.
Begin to add the words to your speaking vocabulary. Leave your ego at home in the bathroom mirror, and graciously accept corrections from those who know. Words have denotations, connotations, semantic bundles and proper idiomatic usage, and you can get any one of these wrong. You can expect to make a few mistakes with the word at first. For myself, I didn't realize that "ahora mismo" was something you only say if you're miffed. Of course, I was miffed when I said it. (My husband should KNOW by now that you do not run off for a beer when there is a lasagna in the oven... but that's another story.)
Listen to people say the words you want to learn. Listen with all your senses. There is a time and a place for every word. Learn to read the proper moments for use of one word versus another word.
This process takes time, and so you must give it time. It's like the box in the attic. You don't find treasures in the attic of a twentysomething. It's grandma's attic that has the good stuff. Stock the boxes and when the time is right, pull out the treasure and enjoy the admiration.