Among the more profitable player options available is the choice to "double down". This can only be done with a two-card hand, before another card has been drawn. Doubling down allows you to double your bet and receive one, and only one, additional card to the hand. A good example of a doubling opportunity is when you hold a total of 11, say a (6,5) against a dealer's upcard of 5. In this case, you have a good chance of winning the hand by drawing one additional card, so you might as well increase your bet in this advantageous situation. If you are playing in a face-down game, just toss the two cards face-up on the table in front of your bet. In either type of game, add an additional bet to the betting circle. Place the additional bet adjacent to the original bet, not on top of it. The dealer will deal one additional card to the hand. In the face-down game, he'll probably tuck it face-down under your bet, to be revealed later.

Blackjack players are allowed to double down for any amount up to the original bet amount, so you could double down "for less" if you wanted. Just remember that you do give up something for being allowed to increase your bet: the ability to draw more than one additional card. If the correct play is to double down, you should always double for the full amount if possible.
The question of when it is appropriate to double down is easily answered by using a Blackjack Basic Strategy Engine