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.

The double angles should be remembered as follows,
\begin{align*}
2\sin^2 (\theta) &= 1 - \cos(2\theta) \\
2\cos^2 (\theta) & = 1 + \cos(2 \theta).
\end{align*}
To know which sign for each, remember that 'sinning' is bad, hence it leads to $-$.
Reason why you should memorize like this is because there no fractions and you can see a pattern!
When you need to compute the integral of say, $\sin^2 (\theta)$, just recall formula and divide by $2$.
