I always observe people, and watch how they play. I try to give advice, unsolicited or no, but I usually preface it with, "Hey X, I notice some things that you could improve - would you mind if I talked with you about them?" And it usually nets a positive response. It's less about shoving advice down people's throats and more about trying to teach them how to play their class, since a lot of people originally fate grinded their class, or only leveled it for speed runs.

Black mage is the biggest perpetrator of this. I see too many BLM's who c ast Fire I, then Flare, then Convert, Then Flare, then don't even transpose to get mana back. This is because the BLM logic of speed runs has become 'Blow all MP as fast as possible" when a basic AoE rotation would help more.