The biggest contributor to Holy Pal neverending mana was using DP very liberally. You don't really notice the 20% reduction if you keep the holy lights coming to keep people topped off. The cooldown is very short and you can use it several times during a long boss fight.
Beyond that, make sure you have a lot of int to get a large mana pool and +crit will return mana.

If you usually run with replenishment, stacking int is a big help. The bigger your mana pool, the more you get out of replenishment and divine plea. If you aren't having trouble with your healing output (which you likely aren't, as a healadin) then you can ditch some of your haste for int or crit. Without gearing for it, you probably already have plenty of haste anyway.

at level 40 before adding in a whole plate set i have almost 3K mana unbuffed and 1.5K health (basically a lock 3 lvls lower than me had more health than i did lol) I never/rarely oom. Stack int out the butt and do crit/haste/sp and you're set... mind you i also have 37sp... not exactly top notch and that is after a Wizard oil buff...

i kinda wish they would add something to lessen the endless pool of mana. make it a little more challenging. even though being a single target healer in a 5-man is a little stressful it is fun... and i'm hoping Shock of Light will help me out with party heals... and maybe a fix to the broken set focus feature? that would help pallys immensely... i remember when i first started healing instances that was a winner. tank is main target next highest threat maker was focus... kept both topped off and it was easier to dispel diseases/poisons and then every now and then switch targets to heal a clothie. please fix this...

wow i went way off subject there lol.

ADD anyone?