Betting to earn rewards from promotions and contests is one of the best way to make a profit. In one gambling site i've lost nearly 0.05
BTC and withdrawed 0.2
BTC from doing promotions. Sometimes you have to be careful because there are promotions that aren't profitable.
Its easy to bet on sports and its more profitable since you only bet between the two teams, i think study the history of the team and it can be a good strategy for you to win though of course the underdog still have the chane to win but it a very low rate. Luck is indeed a major component for you to win in gambling.
It can be more profitable if your sportsbook has a cashout feature. You can quickly secure your profits and avoid losses if the team you're betting on starts to lose their lead in the last few minutes.