MP Conservation
Now, this is something that a ***LOT*** of clerics at all levels fail to realize, but the biggest factor in conserving and regaining your mana is speed and sitting.
The faster you can use a normal heal, the faster you can resume sitting.
When you sit, you regain mana at approximately 500% the rate you do when standing. Look at your MP bar sometime sitting and then standing and compare for yourselves.
I regain enough mana in one single 'tick' of the bars sitting to pay for a lower level heal of mine.
This is also something most people don't know: your MP regeneration is counted as "sitting" the *instant* you press sit. Many people think "oh, it takes so long to sit all the way down and stand up". In fact, that's just something your client does, though. Your MP is regenerating at 500% the second you press 'sit'. I've tested it extensively.
Many people also think "Oh, I can't sit because people are still fighting". This is also a mistake for one reason: you don't need to wait to sit back up. If you press 'sit' again to stand up, the 'stand' animation is superfluous. If you press 'heal' an instant after pressing 'sit' again to stand up, you'll heal that instant. Of course, this is only true if your heal spell is recharged. You can press 'rapid heal' instead if you can't wait for your heal spell. The only real delay is that you have to wait for yourself to sit *down*, but that's a very, very short delay.
Go in game sometime and experiment with this. Sit, press sit again then tap heal quickly. You'll "pop" up instead of standing slowly. Then press sit again when done. Remember, the instant you press sit, you're 'seated'. The animation means nothing.
I keep entire 8-person-parties alive, with almost completely full mana the vast majority of the time, fighting level 70 things in the lost mines with this strategy.
The other key to mana conservation is speed. +dex rings and +cast speed potions (spell/deluxe spell potions) boost your cast speed drastically. I can cast a normal Heal almost as quickly as most clerics can cast a "rapid" heal. Beyond that, though, I'm reducing the amount of time I spend standing. The faster I can get rappelz gold off, the faster I can sit back down. Thus, +dex rings actually give you *substantially* higher effective MP regeneration than +wis or +int.