1. Know your value and stick to it.
There is “being nice”, and then there is the fact that, sometimes “being nice” makes you NOT nice to people around you. This is the case, often, at work, at school, with family.
(this is particularly the case in matters of love, or rather amor and eros, sometimes it’s nice to not be so nice… but this is a digression for another day.)
2. Know your character flaws, with self-knowledge comes a sort of insight that is highly useful.
3. Being blind to what is right in front of you is stupidity of the greatest height.