I love mental rule of thumbs / heuristics that we can install into our brain to avoid getting caught up in cognitive biases or other mistakes.
An easy one that I would expect most HN readers probably do already is: When shopping, always round up to the nearest dollar before even mentally storing it or operating on it. The usual cognitive bias is that many people end up storing the listed price of $4.96 in a lossy manner, as $4.xx, and end up thinking of it as $4 when in reality they could be skipping straight to keeping it as $5 in their head.