The Mathematical Ninja makes ample use of mnemonics to remember how to do just about everything. He was quite upset when Pluto was demoted, because "My Very Easy Mnemonic Just Serves Up Nine" doesn't make any sense.
In particular, the Mathematical Ninja has a creed. A strict set of rules to live by. And I'm not just talking about the Bushido. No, sir. He lives by the creed:
Drink some coffee. Make some more coffee.
It also serves as a reminder of how to differentiate trig functions:
Easy! So you differentiate $-\sin(x)$ to get $-\cos(x)$.
The other thing to remember: when you differentiate things, you tend to times. If you differentiate $\cos(2x)$, you get $-2\sin(2x)$.