There are a few possibilities:
1) Play poker well. This is probably the most popular way, and thousands of people make a significant income just playing poker (keep in mind, this means millions are probably losing).
2) Sports betting. Some players are able to be profitable long-term here. I don't really know much about it.
3) Abuse casino promotions. Although this isn't really winning from gambling and is more just winning free money and trying not to lose it all.
4) Develop an advantage in a casino game. There are a few ways to do this: count cards, learn how to manipulate dice in craps, etc. This is definitely the hardest way.
Good luck!