Does anyone have a useful mnemonic for implementing Comparator?

comparator.compare(a, b) < 0 <==> a < b

I use this simple "substraction" mnemonic:

first - second

So, if first is "less" than second you'll get negative result, otherwise - positive or zero if they are equal.

I am not sure what you mean by mnemonic. However, I have had a very similar cognitive dissonance.

I am very visual, so I use the number line (the one I was taught in grade school). I just visualize the negative numbers as "left", 0 as "center" and positive numbers as "right". That the corresponds to the truth: -1 < 0 < 1