Well I'll put in my 2 cents here as well. As a WHM, I rarely ever run out of mana if a fight is executed properly so I don't know what everyone is on about.

Using my stats, 4.5K MP as an example:
- Casting Shroud EARLY, at 3-3.5k MP. This results in getting more mana in the long run, as your cooldowns are back up quicker.
- Pre Regen'ing targets as mentioned, is better than Medica II. (UNLESS you're in a raid group, and they stack up properly, i.e Titan HM).
- Using cure as a filler to maximize your Cure II procs. Cure II is also generally used sparingly, and usually only on the tank or yourself.
- Never ever use Cure III unless you have 4 melee including the tank, and they are stacked up. Even if it procs. (Unless you really need a big oomph heal on the tank)
If you are still running out mana:
1) your dps are not avoiding avoidable damage
2) the fight is taking too long, blame dps.
3) you are using cure II too much

edit: bard song is OP for later end game raids.