So it might surprise you that I still use flashcards, in my enterprise of learning literary Spanish. I do it for a good reason and a bad reason. The bad reason is that without doing some sort of drill I don't really feel like I'm studying the language. The good reason is that I think -- though I'm still not entirely sure -- that with the right tools and the right approach I can develop my vocabulary a little more efficiently than I could by spending the same time reading or listening.
There are two keys to my method. One is spaced repetition, and the other is context.
I use Anki, a flashcard program that uses spaced repetition. There are lots of such programs out there, with various bells and whistles, but they all use some variant of this simple, sturdy algorithm: you create some flashcards. Then you go through them. If you get a card wrong, you'll be presented with it again when you drill tomorrow. If you get it right, however, it will double the time it waits to present you with it again. You'll see it in two days, instead of in one. Get it right again, you'll see it four days later. And again, you'll see it eight days after that. If you get it wrong, though, the interval for that card drops back to one and you'll start over with it again.
There's some good science behind this: something like this algorithm is optimal for getting stuff into your memory. But there are also some problems. One is that it's easy to get overambitious and create an oppressively large deck that eats up all you study time. Drilling on vocabulary really shouldn't take up very much of the time you devote to your language. A quarter of your time is way too much. So pace yourself. Don't let it shoulder out your main task, which is guzzling down the comprehensible input.
The other main problem is that your brain is very shrewd, thrifty, well-designed learner. It learns exactly what it has to learn. If you have a flashcard that says "crow" on one side and "cuervo" on the other, it will remember that the other side of the "crow" card has "cuervo" on it. But it will only remember it when you're drilling. Meet "cuervo" in a text, and it won't necessarily remember it at all. You may have a vague feeling that you should know it, but you'll have to look it up. And that will leave you with the (legitimate) suspicion that all your drilling is just a waste of time.
The trouble is that you're just learning an isolated factoid. Vocabulary is not a mass of isolated factoids: it's a densely interwoven network of associations. You haven't really learned the word "crow" in English until it has little tendrils of association with a bunch of other words, (bird, black, caw, ominous, fly, croak...). A word out of its web is useless, except for successfully drilling yourself with flashcards. What your brain is doing is learning what's on the other side of the flashcard. What you want it to be doing is weaving "cuervo" into the web of your Spanish vocabulary.
So the "Spanish" side of my "crow" card will look like this, with the sentence where I met it included:
Han pasado cuatro años escuchando el graznido del cuervo.
I don't try to memorize the sentence. If I get "cuervo" right, I've gotten the card right. But every time I answer the card, I say the sentence (aloud or in my mind). I'm stitching the word into the web.
And if I get the word wrong, I don't just start that card over. I start it over, and I create a new card with the same word, but with a different sentence.
My new card might look like this:
Envió un cuervo, pero pronto volvió volando.
(And in the meantime, having refined my understanding of "cuervo," I might change the English side to "crow; raven" -- because "cuervo" refers to either.)
Now we're doing some serious stitching. And by the time you have three such cards, you're no longer having trouble with finding the Spanish word for "crow." You actually know it.
Now, this can be a little discouraging, because it turns out that to keep a reasonable-sized deck you can only add two or three words per day. If you had dreams of building a literary vocabulary in the space of one year, this will dash them. It can't be done. A decent speaking vocabulary, sure. But a real literary vocabulary takes years. It just does.
On the other hand, note that you're not just learning the word "cuervo" here. You're now also learning "graznido," "envió," and "volando," and buttressing the words that you already knew, but maybe not so well as you imagined: "años," "cuatro," "volvió." Not to mention reinforcing the grammar and syntax. You're making the web denser and stronger. Now when you meet "cuervo" on the page it's a real word, a word that means something, a word that has tendrils of association running to other words and turns of phrase.