One of the more frustrating things I've encountered during my math classes are professors that enjoy putting problems on tests that require obscure trig identities in order to solve.
Mnemonic methods work best for me when I have to memorize a large number of formulas. I usually assign verbs to arithmetic symbols, nouns to variables, locations to constants, and other stuff to grouping symbols. It let's me create a story in my head that's much easier for me to remember then trying to do it by route. Of course, you should be able to derive most equations on your own if you get stuck and can’t remember something; this is mainly for things like trig identities, integral tables, derivative tables, laplace transforms, etc…..