Well, I'm not the best healer in the game, but I'm one of the best SP conserving Clerics in my guild. I can give you some advice based on experience.
Timing and Anticipation is a very important factor in conserving SP. The right heal at the right time will produce the most optimal result with the least amount of effort used. There are many times you don't even have to heal at all, such as knowing that there's a long wait between battles; this is when your Healing Aura alone should suffice even for extremely large heals. If many are wounded lightly-moderately and they're near you, throw out a Heal Burst even if it's mid-battle. Heal Bursts are limited in their range, so you should use it every opportunity that comes up because you never know when it is no longer possible to use it in a given situation. Mass Heal should come as a second to last resort, as it is slow but the results are stable. Cure Wound Mass should be an absolute last resort if people are dying way too fast and there isn't time to throw out a Mass Heal before someone dies.
Buffs and Saves plays a very important factor in high level content. Against spells, saves is play a very important factor, even against harmful ones. Even a slight boost in saves, such as Recitation and Holy Aura, reduces the damage the party takes as a whole as it allows evasion types to avoid damage and non-evasion types only take half damage; taking half damage from a spell reduces the amount of damage needed to be recovered in a certain time frame. Against physical attacks, anything you can do to reduce physical damage is a great asset. Spells such as Displacement or blinding enemies with Holy Aura reduces the damage taken by the party, thus making heals with only your Healing Aura or Burst possible.
Participation leads to lowering SP costs. Anything you can add without exhausting yourself helps. Even if it's a 1d3-1 fist you're throwing out, it's still an extra 1d3-1 bonus to the overall numbers. The faster something dies the faster it can no longer do damage. If you can vorpal, vorpal; if you can paralyze, paralyze; if you can hamstring, hamstring. Participation has risks, but if you can overcome those risks then this will become one of the biggest benefactors to your healing career.
Communication will save the world and it will save you. If you do not speak up, then you will not be heard. If you don't ask, you shall not receive. Basically, if you want or need anything, speak up as you are a member of a team. If you want a monk SP discount for buffing, ask for it. If you want the Rogue to stay back for a little while, say so. If you want someone to have Heavy Fortification, tell them. If you think someone does not have sufficient HP, tell them (in private if required). If you think the group should gather around you for a heal, say so. The less you say, the harder it will be for you. It makes no sense to be anti-social in a team environment.
Versatility helps in reducing the amount of SP required in many things. If you can disarm traps, it reduces buffs and healing required. If you can stun, it reduces damage the party is taking and increase the damage you're giving. If you can crowd control, you can turn things in your favor and reduce the damage the party takes. If you can hamstring, do so and you will conserve SP. Anything you can add to the party is always a welcome addition, even if it's not the best, even if it is just Divine Intervention.
Levelheadedness is something that should be cherished. A person only has so much energy and endurance both physically and mentally. The less levelheaded you are, the more mental energy you will consume and the more rash decisions or inattentiveness you will produce. Try not to excite yourself and think things through, this way it allows you to calculate and time things more accurately. The more mentally exhausted you are, the more mistakes you will make and mistakes cost either your SP or the party members so keep that in mind.
Party Morale is very important. The more you joke around and create a light/relaxing atmosphere for those around you, the easier it is to both communicate and remain levelheaded. Throw jokes if you can, talk about things you're interested in, talk about subjects outside the game, anything you can do to take people's minds off potential troubling matters will only help you in the long run. If you can crack a joke while giving important instructions, people will appreciate it more and not take it extremely heavily at heart. Remember that well done tasks are usually done when people are happiest; a world class masterpiece is not something made with anger and frustration. So, if you can keep the mood up, do it; even if it makes you a weird person (better to be remembered a weird person than a frustrating person).
Well, these things won't make you the best healer, but it'll definitely make you better known, respected, and appreciated healer.