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.
Blackjack naturally has a mathematical percentage that favors the house, although the payout percentage varies by the skill level of the blackjack player. Online games use random number generators (RNGs) to ensure their odds match the payout of land-based casinos.