Alright, we all know the easy way, donate, lol. But here's a simple way to do it without spending your allowance...
Conquer counties. A lot of counties. I'm currently at 53 and wish I had twice as many...here's why:
Sell your human troops. Yes, ALL of them! Keep a few thousand bumps, but the rest go in the sell pile. You know they suck, I know they suck, so why not sell them to the suckers that buy em? Nearly every human troop sells for more than a lizard does, and who does like the long-tonguers? (Hush, ladies).
Alright, so you got some counties, they're all producing, what now? Now we streamline...and this is the tedious, omg takes forever, arthritis & carpal tunnel sydrome causing part...ok, so you've got some building managements, right? If not its still doable...but kinda like the last chick at the bar before it closes, doable...so downgrade a building (or two if you can) in each county, cellars first since they suck, and they go down faster than a virgin on prom night. Once you've got one down, (a cellar, not a virgin) build a workshop in its place. Then upgrade said workshop until its level 10, downgrade again...and repeat. After you've run out of cellars, switch to barracks, they're faster than the others. Leave a few human troops buildings for upgrades and to produce bumps, and crank out catapults like turds after taco bell.
Now, here's the WIFM! (What's In It For Me) Every workshop will generate about 20 gems a week. I know, you're thinking, 20 gems for all that work is some straight bullshit...and id agree, this is where you refer back to my first statement...conquer A LOT OF FRIGGIN' COUNTIES! Even if you were to have a measly 30 counties, with a piddly and unmotivated count of only 4 workshops in each county...you'd be raking in 2,400 gems a week of catapults alone...and that's not even taking into account your fast recruits you use your mana on! Got chubbage? It's not polite to point.
Moving on...Now, to maximize the level of awesomeness of your cat slinging capabilites, id recommend you always have some workshops with the max load of cats possbile being built. Use fast recruit on one until its at 200hrs, then leave that shit alone! I know, you youngins want that whole instant gratification feeling, but trust me, slow and steady always wins. When mana fills up, blow it on another workshop that's recruiting max, and so on. In a little over a week, you're gonna be ass deep in catapults, with more plopping down every day. The more vigilant you are about spending mana when its available, the higher your payoff will be. Enjoy, take care, have fun, be safe, and when in doubt, wrap it twice!