Gyms, gyms, gyms and more gyms! As in: Defend them! Once recently, I had 5 Pokemon in 5 different gyms. Well, I got nothing to show for it when one of them got knocked out of the gym, but that's the way you gain coins.
Best way to get those coins without having to use a mass amount of potions and revives would be to place your best pokemon in gyms that are not found in popular areas. Gyms get attacked several times every hours in downtown of a city but if you make a visit to go see family in their rural town, taking a gym there and placing your pokemon in that gym could easily give you your full 50 coins just from that one gym!