Remembering that $\sin^2(\theta) = 1/2 - 1/2\cos(2\theta)$? How do you remember this for integrals?
It doesn't seem obvious and I can never remember it when I come across it in integrals.

HINT:
I remember the identity by first remembering the addition formula:
$$\cos (x+y)=\cos x \cos y-\sin x \sin y \implies \cos (2x) =1-2\sin^2 x$$
