You should some across some items from which you can learn it in your travels. Teach it to everyone in your party! Although it costs 10ap to have it equiped, the benefits are huge. Not only do you get a steady income of free hp during the battle...but...well, first things first:
1) Teach Eiko or Dagger the "Boost" ability, learned from Pumice Piece (obtained by killing Ark in Oeilvert). This ensures that summon animations always play in full.
2) Go into battle, and summon Madeen, Bahumut or some other summon that takes a nice, long time. The Auto Regen will continue to function throughout that time, thereby giving your party...err...lots of hp!
Umm, that might all sound a bit obvious, but lots of people I mentioned it too thought it was cool. So I thought I'd post it. Yup. ^^;;