A basic betting strategy is simply to double the bet after losing a hand, also known as the Martingale system. This can go for multiple hands in a row until you win at blackjack.
Classic Blackjack is the best option for those looking for the lowest house edge in online blackjack games. For instance, if you've had a total of 9, 10, or 11, you'll be able to double down on that total, as long as the dealer has a soft 17.