https://www.casino-strategy.com/ Here you can find some nice strategies for roulette, slots, and black jack. With nice explanations about that. There is more sites about this question, forum talks, videos on youtube. I read a lot about this things, and I tried some strategies that I found interesting in my opinion. YouTube is also nice place to see how some people win, people film it and you see some strategy from first hand...
Martingale is universe, I like to play progressive and that is basically martingale, just I don't double bet every time, sometimes I rise bet even much more. and when I`m on high bet sometimes I like to repeat bet. Cause many times I saw and get two good ones in a row. So I like to play progressive and on feeling when to rise, or lower bet.
I advise you to read and check all this things I told you about, and after that to try some mix of all that. Combine good things in your head and that is it, sometimes you will lose, sometimes you will win, its gambling. Just try to win a bit more.