Delta, after having done some "rucking" both for unit training purposes and on a few courses (which for an infanteer is basically inevitable), here are a couple of things I can offer from my own experience which has been bettered by practical regimen and the advice of a handful of thorough-bred infanteers.
1) The human body (as far as I know) tends to naturally lean forward to compensate for various loads carried on the back. Essentially, the heavier the load the steeper the angle of the back and of-course the lighter the load, the more upright your back positioning will be. Furthermore, I have not seen a person marching on a 90 degree angle (well maybe a couple of times in the movie: The Devil's Brigade, sneaky suspicion that Hollywood had something to do with that) on a 13k or a 24 k for that matter.
2) When loading the ruck up for the deed, try and stuff bigger and heavier items on the large center of the ruck and fold the items evenly so that you don't end up having your socks and undies hanging out one end and your Gortex jacket and fleece on the other end cuz that will chafe your shoulders and armpits and believe me they will chafe you good. Same rule applies to the contents of the sleeping bag.
3) Tighting up each strap will be extremely beneficial as it will pack your equipment tightly and closely knit together. Make sure that you tighten the shoulder straps only to the point of comfort. Very loose will hurt your back and too tight will hurt your shoulders. I'll let you figure that out on your own.
4) Tracker23A has a good point on the use of the waist belt, after-all it is there for a reason. My first ruck came with the chest belt too so they both help. I personally have used the waist belt very little but on longer humps I have used the waist belt on and off for tactical/friendly situations.
5) Try and keep your pace at a constant and steady tempo (usually dependant on the mood of Sr. NCOs, scattered staff and situational readiness) and avoid stomping, instead attempt taking softer paces. I have seen several folks tired halfway during a BFT or a LRP because they started stomping or dragging their feet on the ground.
6) Finally, there is no such thing as a perfect hump and you will be miserable one way or another. If the chaffing doesn't slow you down, the blisters, the weather or some other earthly anomaly will. Good news is that they do tend to (or seem to) become easier with a steady diet of regular practice, constant hydration and the phenomenon known as the "WILL". Just keep on keep it on! Hope that helps
Cheers!