Frankenslot it!
Go to Wentworth's and pick up recipes / enhancements that combine Heal / Recharge / End Redux so that you max out healing (realistically, about 97%, or 3ish white SOs before diminishing returns set in) and recharge, with a side of end redux thrown in.
If you don't know much about Inventions, head over to the Invention forum -- there should be a handful of guides that will explain things better than I ever could.
Looking at just SOs, you'll want 3 Heals, and then either 1 Recharge and 2 End Reduction, or 2 Recharge and 1 End Reduction. Go for more recharge if you don't have any endurance troubles. Go for more end if you do.
Once you start looking at IOs, it becomes a lot more complicated. Personally, I really like the Doctored Wounds set, so I slot that whole set into my HA. I also have that whole set in Heal Other and Regen Aura, and I have 5 pieces of that set in Adrenaline Boost (the 6th slot is an End Mod).
I'm no expert (by any means) with IOs, though, so look to someone else for better IO slotting advice. Ideally, you'll want to max out the healing, the recharge, and the endurance reduction, and sets will let you do that fairly easily.
Bye, everybody!
*Champion*
With SOs? 3 heal, 1-2 recharge, 1-2 end redux, depending on your end usage. More end redux if you're struggling, less if you're not.
My empath has it 5 slotted with doctored wounds to get the recharge and healing bonuses, and the 6th slot is a heal/end hami-o from back in my raiding days.
For IO sets I also use Doctored Wounds though usually I only use 5 of the set in both HA and HO. As far as the common single aspect IO's the slotting you use for these is basically the same as with SO slotting ... 3 Heal with recharge and endred to taste.
So far agree with what everyone else has listed. I currently have it 6 slotted with Training IOs, 3 heal, 2 recharge and 1 end rdx. I am looking at either doing Doctored Wounds or Numina's as a full set, although I can't really afford Numina's at the moment.
Coming back out of the woodwork after a couple years away from the game...
not adding anything to slotting advice, because everyone else already covered that,
just want to say that Emp/Dark is an excellent team build. You have "decent" AoE damage, but really, your biggest contribution from your secondary is going to be the -tohit debuffs and controls you can provide.
No
Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo
I think you underestimate our fools, sir.
Why /duel is a bad idea
Hmmm, I must be the odd guy here. I slot 4 Numinas in HA and a heal/recharge DW and a heal IO
Dont know yet if I will slot DW or numinas in HO yet. I repected and that power is empty at the moment.
By the way, this is on my ill/emp. I respected to pick up haste.
Also another tip is try to get a couple sets of Healing IOs in different powers. I think I have at least 4 in HO, AB, RA and maybe in Health(can't remember, has been awhile since I looked at my build), because you get the overall healing bonus that you can use from up to 5 sets.
For example if you slot 4 Numina's in 5 different powers I believe you get a 30% bonus to healing powers. (6% from slotting 4 in one power)
Coming back out of the woodwork after a couple years away from the game...
Basic IO slotting has been discussed.
As for sets... it depends.
With 6 slots for a full set, your best bets are Doctored Wounds or Harmonized Healing. Doctored Wounds is better for Recharge, Harmonized Healing is easier on the Endurance usage. Using level 35 enhancements - and not considering adjustments from other sets in the build -
Harmonized Healing:
Heal: 26.1%
End use: 6.66
Recharge: 4.87 seconds
Set bonuses:
1.5% Endurance Recovery
1.65% Confusion Resistance
3% Global Heal Enhancement
1.88% AoE Defense; 0.94% Fire/Ice Defense
1.88% Toxic Resistance
Doctored Wounds:
Heal: 26.2%
End use: 7.92%
Recharge: 4 seconds
Set Bonuses:
2.2% Terrorized Resistance
1.26 Fire/Cold Resistance
4% Global Heal Enhancement
5% Global Recharge Enhancement
1.26% Toxic Resistance & 1.26% Psionic Resistance
Taken in a vacuum, I'd prefer the Harmonized Healing set, for the effect on the power itself.
As for Miracle or Numina's Convalescence... Unless you're building specifically for some of the set bonuses in those sets, they aren't worth the cost of procuring and crafting them. And I think you would be tarred and feathered if you even considered slotting one of their Unique enhancements into Healing Aura. (Or at least, you should be!)
I'm not good at Frankenslotting, so I'll let someone else work out how to get the most out of the power with less investments of slots.
-This Space Intentionally Left Blank.-
Going to jump on the bandwagon and say doctored wounds. While doctored wounds isn't the only decent heal set, the set bonuses (in my opinion at least) do more for an empath then the bonuses of the other sets. If we were talking about slotting O2 boost instead of healing aura another set may be preferable in the context of the powerset as a whole. Anyway, key reason I say doctored wounds, it has a global recharge boost. As an empath you have fortitude, the 2 auras, and adrenaline boost that you want to get as much recharge as possible for. Thats almost half the set that is hungry for more recharge, and it's not like extra recharge is really going to hurt the rest of the set.
And I'll also second Psyonico, Emp/Dark is a very nice team build, although I may be biased as my main is an emp/dark :P
Who cares how you slot it? Just Ctrl-click it once, make sure it has that nice green circle around it, and heal!*
*Advice in this post is probably bad, and should not be followed under any circumstances.
@Roderick
Well, you could slot it with one level 50 Accuracy IO, two heals and three damages.... but I always preferred a full Positron's Enema set. 
Wow, thanks for all the responses and great humor! I've got some reading up to do!
Hi everyone. This is my first post. I have recently returned to Paragon after about a 2 year break and am having a really great time. I put together an emp/ dark defender and it's fun. My problem is I felt that healing aura just isn't that strong. I have heard that anymore than 3 enhancements is redundant. So- if you had maxed out slots with healing aura, would would you take? And if the emp/ dark build is goofy just say so
Thanks