Your Double Down Bet
To maximize the effectiveness of doubling down and take full advantage of doubling, you should always match your original bet when you exercise this option. Many, if not most, casinos will let players double down for less than what they originally bet. If you're not sure about your chances, or you're worried about your bankroll, it's an option to consider, but it somewhat defeats the purpose of doubling down.
If the thought of doubling down puts you in a cold sweat because the amount you'd have to put up moves you out of your comfort zone, you're probably playing at a table with stakes higher than what you are comfortable with in general. Do yourself a favor and drop back to a table where you can take the best advantage of this powerful player option.