I can't believe there are people out there who think Martingaling is a strategy that will yield a positive expectation over the long run. Hate to burst anyones bubble, but the Martingale system is -EV and you will lose money playing it. Back in the day you could bonus-whore the online casinos playing games where the house has a slight edge and the bonus would give you a positive expectation, but those days are long gone and they won't be coming back.
You can sometimes have an edge in blackjack, however most people will never have an edge and those who do will have a very tiny one. Sports betting offers greater edges than blackjack but long term you'll likely only have a 5%-10% edge if you're good at it. The best way to make money in gambling is via poker, however most people are far too results-oriented and emotionally unstable to cope with the swings associated with the game.
Playing poker for income is a very stressful job especially now that there is so much information available and people have gotten so much better at the game. Anyone who thinks you just click buttons and print money is dead wrong.