A flash card is a little piece of paper, the size of a business card, which has on one side a new word in a language you're learning, and on the other side a word in your mother tongue. 

How do I make a flash card ?

Take little cards or better, make them out of thin paper (so that that you can pile up more, durability is of no importance here). Always have a blank stack with your language method, dictionnary, radio, or whatever tool  you use to learn your language. Whenever a new  interesting word  appears, write it down on a blank flash card, find out its meaning and write it on the other side. At the end of the gathering session, you make a little decks with your flashcards, and put them in a little plastic pocket.
Alternatively, you can use my free flash card software if you have Access.

Where can I get ready-to-use flash cards ?

If you need to build a general vocabulary, buying a stack of printed flash cards is an attractive alternative. They are stronger, clearer and save you a lots of time. The best general publisher of flash cards I know is Visual Education. Their typical package contains 100 flash cards with one word and its derivative on each, for at total of over 3'000 words. I used the russian stack with great success, and friends told me great things about the italian one. A good buy for 10$.

How is it used ?

Whenever you have time (see below) you take a deck  and begin with one side for (example,   french to english) and try to discover the word on the other side. If you guessed correctly, you move the flash card to another deck, if not you put it at the end of the original deck.
Then  drill you skills in the other direction (english to french) in the second deck. The words you successfully guessed go to a third deck. After a week you can take this third deck  and go again from french to english, and so on until you know each word perfectly, or just decide you know it and then put the terminated flash card in a cup over your fireplace.

When is it used ?

The best part of this method is that you don't need to allow any special time to learn your flash cards. Just have your decks on you at all times during the day, and whenever a small moment appears, take your decks out and drill a few cards. Such hidden moments include :

• Waiting for an elevator and inside it
• Waiting in line at some desk, museum or cinema
• Commuting in public transports
• Waiting on the phone
• Waiting at the restaurant

Why is this method superior ?

• It's free, versatile and easy to use
• You don't need any specific time to drill your new vocabulary, all the time it takes is already here, hidden in your day
• It's far better than word lists, because you can break it up in any minute and then group the words by their difficulty, they don't stay in the order you wrote them down
• The moment you find a new word you'd like to learn, all it takes is to write it down once, and that's it ! You're ready to drill it until you know it, and you cannot "forget" about it. It will always come back on your deck until you learn it to get rid of it!