The Calculus prof I had gave us 'the smiley face rule'
Draw a smiley face and have the eyes with plus signs- positive second derivatives have the curvature of the smile, draw a 'frownie face' with the eyes closed and sad- use minus signs- I only remember this mnemonic because he was such a dour, and mean, SOB, and it seemed so out of character for him.
On a test he asked "What is the curvature of the following function of x, at x=5?" I answered "The cuvature is sad" as a joke- he did not find it funny...