The next time you butcher a cow or pig, introduce the horses to the blood and carcass. This is a good start. Depending on the horse it should go pretty quick. It's been my experience they either tolerate it or they don't.
As far as packing goes, if there was one thing I would be sure of, is making sure the loads are as well balanced as possible. An unbalanced load can cause a pack to spin which can lead to major rodeos! An other thing that can play into the picture is the compatibility or the pack animals. Some critters just can't abide having some animals behind them. One time I was leading a couple of horses and my saddle horse, which I had been riding for weeks with no problems become a snorting bronc! To make a long story short, all I did was swap positions of the horses being lead, problem solved. My saddle horse had serious issues with the lead pack animal, so when I moved him behind the other horse it solved the problem. So be aware of things like that.
Enjoy your pack trips, I love 'em, but they can be a pain until you get some experience.