I used to run the dehydrator to make for training treats for Denna...
Slices of beef heart with slices of apple and sweet potato - when dry enough to handle comfortably mix them all together and feed as needed.
The heart makes it all smell like yummy beefy goodness, and the apple and sweet potato just add 'bulk', so you can treat more often.
Denna will eat carrots, apples and sweet potato in any given amounts, though.
Don't use liver - 1) it takes longer to dry and 2) feeding too much will cause runny poo's
You could look into freeze dried proteins (not liver) - "Whole Life" has some chicken bits and other freeze dried stuff.
Whole Life Pet Products (not cheap, though)
Another item we use is just big kibbles - or her usual dinner kibbles. Not quite as high reward for during class, but works fine when at home practicing.
Solid Gold Wolf kibbles are a big size, and pretty good from a nutrition standpoint, too.
There are also freeze-dried whole-meal nuggets out there that work good for treats too.
Something like these:
http://www.onlynaturalpet.com/products/Only-Natural-Pet-Freeze-Dried-Nibblets/999325.aspx
or these:
http://www.onlynaturalpet.com/products/Only-Natural-Pet-Green-Beef-Tripe-Nibblets/999350.aspx