When I saw the equation referred to as (*), I had a flashback to those problem sets with *hard and **harder problems. ** problems often required some real out-of-the-box thinking. I wasn't always able to solve those, but it was so satisfying when I did (usually after an hour or two of struggle).
I also like labeling formulas with playing card suits. I think they are easier for the reader to distinguish when they are looking back for the labeled formula.
A while back I had the idea of marking erroneous formulas with a red spade. I'm going to try doing that again because it's hilarious.
