Play matches, lots of them. Typically I have been getting around 5-600 coins per match, even if I draw. Do 10 matches, you've got roughly 5000 coins (assuming you don't spend any of them, which inevitably you will)
You could do things like buying and selling on certain items, but I personally didn't buy the game to sit there for hours selling on items and looking for certain cards. And like I've said before you can get very decent players, for hardly any coins, so even if you just play matches and get 500 a time you can build up a strong team very quick.