Martingale is an aggressive strategy where you double every next bet. It works sometimes, but not forever, sooner or later you stumble upon some losing streak and you lost everything you have.
If you wish to win in gambling you need to play aggressive and you need to bet higher after you lose. Talking about this in few lines is impossible, cause everything depends from the game you play, your bankroll and of course what kind of odds you prefer to bet on. I tried many strategies and non of them works entire night, that`s why I advise to mix your strategies, to change them often, to change games you play as much as possible, same like you need to change odds you bet on, as much often as you are capable. Try playing with bigger amounts and lower odds, then try to hit some big odd with small bet, you need to have some feeling about what will happen next and try to predict the outcome.
That's the main reason for every one to lose, it does not need to do so, there is odd to be used as martingale base rather than bet stake. And "double every next bet" is a for sure lose, with combination of odd, and increase enough bet size it may be as less as 0.01% is enough to recover loss and earn some gains. Another case where majority of people don't realize that with the same capital betting on 50% chance will cause the whole balance to be lost easier than 10% chance.
Also, the statement "you will lose in long lerm" is correct but not 100%, the question here is how long it is, "standard long" may be a week 24/7, but it may be extended as long as 1 year or 10 years. If you managed to have a strategy that can stretch 10 years 24/7 straight.. that could be enough since you may have option to quit with profit during the time.