here are my recommendations in addition to what others have posted here already.
use low level elemental nukes. it's a given that you shouldn't go in to battle (with a nin tank) and use level 3 and 4 spells right off the bat. that would make YOU the main tank of the party. instead, use level 1 and 2 spells. what i like to do is this:
level 1 spell > level 2 spell of the same family > level 1 spell used in the beginning
the level 1 spell will recycle (barring slow) by the time you finish casting the level 2 spell so you can use it again.
illustrated with blizzard:
blizzard > blizzard 2 > blizzard
and then i wait for blizzard 1 to recycle before doing this again. i like this pattern because it does enough damage to be effective, doesn't pull hate like casting a blizzard 3 would, and saves mp. i do this regardless of who is my tank (pld or nin).
help cure. especially if your back line is rdm/brd/blm. i know from personal experience that an rdm is under major pressure to do their job (refresh, haste, and dispel) and keep cures going. when a big hit lands on the tank while the rdm is in the middle of casting refresh or haste, they will follow up with cure 4 and they will pull hate, especially since rdms who are optimally equipped will have NO -enmity gear before level 72. and even if you have a whm, help with cures so that the whm doesn't pull hate as well.
ninjas have shaky aggro control at the beginning of any battle but towards the end of the battle (once the mob is at half hp or below), the hate will be solidly on the ninja. with a good tank, it would be difficult to take the hate off of a ninja near the end of a battle you could probably let loose 1 or 2 level 3 and 4 spells without pulling hate.
