I've used flash cards a lot to learn Korean words, and it doesn't take long to memorize words well enough that I can recall them very quickly. It's especially easy to go from English to Korean - I can recall the Korean word very quickly.
However, I feel the knowledge I gain is quite shallow for many words, and often I fail to add that vocabulary to my active vocabulary (words I can actually use). I can recall these words instantly when I use flash cards, but I can't use them when speaking or writing the language. It seems I'm associating the Korean word more with the English words on the card than with the actual meaning.
What activities can I use to deepen my knowledge of words that I've memorized using flash cards?