I see this question asked a LOT. Here's how healing enhancements stack, with an example:
1) Your best item and best enhancement stack additively. Example: Life Magic 4 (40%) and Greater Devotion 6 (40%) add to an 80% boost. Convert this to a multiplier (so +80% is x1.8) - this is your enhancement factor. Note that clickies such as Superior Efficacy 8 or Superior Ardor 6 count as your item even if you've used the clicky then equipped something else.
2) Your metamagic feats stack additively. Example: Empower Spell, Maximize Spell and Empower Healing give 50%, 100% and 50% respectively - total of a 200% boost. (For Heal and Mass Heal, only Empower Healing works, and I know very few Clr's or FvS's that have both Empower Spell and Empower Healing).
Again, convert this to a multiplier (+200% = x3.0) - this is your metamagic factor.
3) The enhancement and metamagic factors stack multiplicatively. So in this case, 1.8x3.0 = 5.4 times the healing output. Call this the combined factor.
4) The combined factor stacks multiplicatively with every source of Healing Amplification the target has, except the item named Finger Necklace (this stacks in strange ways, probably due to a bug). If the target is a Warforged with Healers Friend 2 (x0.7), and a 10% and a 20% boost to Healing Amp on their DT docent (x1.1 and x1.2), then this factor is 0.7x1.1x1.2=0.924, multiplied by 5.4 (from part 4) for a grand total of 5.4x1.1x1.2x0.7=4.9896.
5) Finally, this stacks multiplicatively with any critical heal that may be rolled up. If you have a total of 225% crit heal intensity (base 150%, +50% Prayer of Incredible Life 2, +25% Greater Healing Lore), then your non-crit heals hit for 4.9896 times their base value, and your crit heals hit for 2.25x4.9896 = 11.2266 times base.