I also recommend getting your dog used to the pack before the trip and making sure it doesn't rub him raw anywhere. It's never been an issue with my dog but it would suck to find out it is an issue when you are 5 miles in.
Rough wear makes good packs. My dog loves the water and creek crossing so I double bag his food to keep it from getting soggy. Usually each meal goes in a sandwich bag. Then I split them in half and put them into gallon size freezer bags. One in each side to balance the weight. Also if you put one beer in one side you have to put a beer in the other.....