I use a pack that separates from the harness/vest. Once I got the harness fitted I don't have any problems keeping the pack upright.

I took my dog out on a horseback ride with his pack. I didn't put too much in it because I knew he would be running a lot, but I wanted to see how the pack would really do. We rode for about 13 miles. My dog kept right up with us running back and forth, jumping over logs and around bushes and trees and not once did I have to get down to adjust his pack. Everyone I was with was surprised how well the pack stayed on.