Answer Nick replies
I begin by inspiring myself with some of the images to be found online of folk carrying remarkable loads. Start by sketching a few ideas based on the inspiration you find. I soon land on the idea of a squire carrying spares for his knight, including a horse. I sketch that idea in SketchBook Pro and then bring it into ArtRage (select Import Image to Layer).
Once I get the general proportions and pose working better, I add more elements on top of the horse. I use simple shapes like boxes and cylinders that I can make feel solid. It all adds to the feeling of weight. I draw the figure bending forward with the burden on his back, a strained look on his face. I add further to the impression by having his knees struggle to stay facing forward. Perhaps have him labouring uphill, or up some stairs. I contrast his body angles with clothing that hangs straight down.
A combination of awkward posture, larger, solid volumes above him and a straining expression can all add to the impression of overloading.
Once I find images showing how much some folk actually carry, it gives me licence to push my sketches further, ending up with my overburdened squire.