Some players seem to go months without hitting a double fault. Impressive as that may be, it doesn't necessarily mean they have effective second serves. In fact, it could be a sign of a common problem: passive, overly safe second deliveries.
Though few players actually try to win the point with a second serve, the aim should be to keep your opponent from getting the upper hand. You can do this by occasionally taking a chance and hitting your second ball harder or placing it closer to the lines than usual. This will:
Keep the receiver guessing: While consistency is an asset, predictability isn't. If you give your opponent the same ball every time, even players with average returns of serve will begin to take advantage. Mix it up to keep them off balance.
Keep the receiver back: Most players step in to receive second serves, but if they know in advance that you'll be sending over a cream puff, they'll move in even more.
Although your gambles may mean an occasional double fault, the reward will be more effective service games.