The first one is betting games. Each bet that equals to 1$ or more you place grants you 5 coins, no matter if you win. If you bet within defined situations, you earn 5 coins at the end of the game.
Also, another way to earn coins is by winning a game. When you win a game, you will be granted coins regarding the skin values you have earned. So for example, you will earn more coins if you win 200$ instead of 20$.
The last way to earn coins is the affiliate system. You can get more information about that by clicking the "FREE COINS" tab in the homepage. The affiliate system is probably the best option to earn coins, if you have a large community.
Also there are some seasonal events where codes of coins are given. Try to catch one for some free coins!