1) You can click regen off. If they put it on you or it's staying on before a pull, turn it off and tell the healer that it's for his own benefit-- the mobs will zerg him as soon as you pull if its on.
2) Stoneskin is your friend. I'm not very keen on the scholar, I usually go whm but I may phase into AST soon, but even in Cleric Stance, stoneskin will block its full damage. Thus, if you tank needs a "heal" and you can't get out of cleric stance, pop a stoneskin to buy you some time and quick cast it if you need to.
3) 50% while it varies from healer to healer, 50% is my magic number. If the tank is at or below 50%, I will pop out of cleric for a heal, a regen, and go right back to dps (if the fight seems like it will last that long.) 50% assures I will have enough time to get out of cleric stance, cast my heal and assures the tank has enough HP to survive any random spikes of damage.
4) WHMs don't get Shroud of Saints until level 38 which is insane. If you're in content lower level than that, realize that your whm may not be DPSing because he/she knows that we don't have a mana regen ability yet. Can you still do it? Of course you can, but it makes MP management a lot harder. =/
5) (WHM only) When you're low on MP, use cure instead of cure 2. Yes, it heals less, but the chance to pop Free Cure for a free Cure II is there and that could be the ticket you need to get through to your next shroud of saints or ether.
6) (WHM only) Fluid Aura can be a tank's worst nightmare and shoot mobs out of DPS AoE. It's not that big of a problem in high level content since the whm is mostly going to be using Holy to dps, but healers that use it in low level dungeons improperly can do more harm than good. I only use fluid Aura in 3 situations low level: 1) I have an undergeared tank and he can't handle all the mobs and I don't have repose yet. 2) The DPS are being jerks and waking up my sleeped mob and the tank just can't handle the damage. 3) A mob has wandered over to me and fluid aura will shoot him back within range of a tank. In high level areas, bosses are immune to the push so it's fine to use on bosses and the like.
7) (WHM only) Why NOT holy? Yes, it costs a lot of mp. Yes, you need to be in cleric stance for it to do any real damage, but it stuns ALL the targets in the area for 3 seconds. This interrupts casting and skills making it easier for everyone not to take damage and the 3 seconds is plenty of time to cast stoneskin on the tank before your next holy which you NEED to wait before casting again anyway or your stun won't take effect. Even if you're only comfortable casting it once or twice per pack of mobs, that's enough to really get your tank some breathing room while there's lots of mobs on him/her.

That's all I can think to add atm.