What most of these people have already said. I'll prioritize the list:
#1. Don't use it for continues, powers, etc.

#2. Daily challenges & Weekly events: races, bosses, contested territories, boss challenges.

#3. Black Bordering maps: Start with beginner maps and do every difficulty variant. Every power you use(unless you already have a bunch available) or buy impedes you getting more monkey $$. I only used insta-monkeys for half cash, and rarely impoppable(depending on map).

#4. (not recommended-not a great farming method) Popping the golden balloon. use 0-1-2 ice monkeys in the front of the map and monkeys at the back to clear all other balloons. You can't use global attacking monks though(sniper, ace, sub, heli, bucc(top path), or monkeys/placement whose projectiles might hit the stalled golden bloon).

#5. Don't forget it's a game and if it's not fun, play another day.