Harnesses

My Favourite Sled already to go.

Rope trace and Bandolier (Cross-over) Harness

Using a double bandoleer and caribineers with a rope trace.
Click for larger image. 50k For traversing a relatively flat landscape, take a hint from Artic explorers and use a simple loose rope trace. It's just a long rope tied between the sled and your harness. I loop 16 feet of rope to make an 8 foot trace. If you want something from your sled or if the sled tips over, it's not a problem to just stop and walk back to the sled.
A rope trace combines nicely with a couple of cross-over bandoliers. Each bandolier is 60 inches of 2 inch wide webbing made into a long loop, crossed over the chest and attached to the trace with a carabiner. Some like two traces each affixed to its own bandolier. When travelling over ice, I use a quick release on a single banolier and a single 16 foot trace.

Belt Harness

A waist belt with shoulder strap to prevent the belt from being pulled too low
Click for larger image. 50k A 2 inch wide belt harness puts the strain right over the hip bones where it is hardly noticeable. Because the strain is at a downward angle, the belt will slip down to your thighs if you don't use cross-over shoulder belt to keep it in place. (Similar to the WW1 Sam Brown belt.) This type of belt can be used with either a rope trace or semi-rigid poles. If travelling on ice a quick release rather than ladderloc is a good idea.

Semi-rigid Trace

Hilly country can be problematic, especially on crusty or iced over snow. At some point during a descent, the sled can run forward right over the back of your snowshoes or skis. This will cause a not so pleasant face plant in the snow and perhaps a damaged muscle or tendon.
All plastic or all metal tubing are problematic. Plastic breaks and metal can act like a spear in your back. Therefore my version is nothing more than 6 feet of rope trace inside a piece of plastic tubing. The rope still takes the strain and the tubing stops the sled from running you down. Try to keep the attachment points at the belt and the sleight as tight as possible. The more "slop" the less control. Nylon rope stretches too much causing control problems. Polyester rope (finally a good use for poly rope) doesn't stretch and is water and mildew resistant.
Better cornering and tracking can be achieved by making an X between you and the sled with the tubes. Affix the tubes at the centre of the X with shock cord or the like. Don't put a hole and pin through them as they will break at that point.