Oh boy here we go, this is one of my favorite topics.
Simply put, posture and body movement is probably the most important aspect of kart driving. The driver is the largest piece of ballast on the whole kart, often accounting for nearly 50% of the total weight. So of course where you place that ballast has a huge affect on the handling of the kart.
In order to minimize any ill handling from your body position, it's important to have a very stable posture. The drivers you watch in WSK races are all firmly back in the seat, with their arms straighter. Having your shoulders back in the seat provides a much more direct weight transfer from the seat to the seat stays, and in turn, to the rear axle. Having strong arm positioning will help keep your steering inputs much smoother and more predictable. Your emphasis should be on pushing the wheel in a turn with your outside arm, rather than pulling with your inside arm. This will force your body back into the seat further and press more weight through the seat stays. Also, having your arms at a straighter angle will eliminate the tendency to over-drive the kart. When your arms are straighter, you are steering more with your shoulders rather than your biceps. Your biceps have more leverage and therefore it's much easier to aggressively turn the wheel. Using your shoulders rotates the wheel in smaller increments, eliminating choppy steering inputs.
When I drive, I tend to keep my shoulders firmly pressed on the outside of the seat, transferring as much weight as possible to the rear axle. This will help plant the outside rear tire and lift the inside rear. My head generally stays level with the racing surface. Here are some reference photos:
Here you can see how straight my arms are, and how my upper body is already tensed up, ready to lean to the outside for the next right hand corner.
In the above photo you get a really good look at how my shoulders are leaned to the outside of the seat, letting the inside rear wheel unload.
I was coaching at the last Route 66 Series race in New Castle, and one of our Yamaha drivers was struggling with driving posture. He's tall so any issue he has with posture is amplified with his high center of gravity. One of the tricks I tell drivers who are struggling to be smooth on the wheel is to move their hand positioning to the top of the wheel, around 11 and 1. This will force you to drive with your shoulders more and slows down your steering inputs. On top of that, I also had this driver switch from pulling on the wheel to pushing on the wheel and forcing himself back into the seat to transfer weight. With these two changes, no adjustments to the chassis, we picked up .5 in one session. The kart handling went from too loose to completely neutral. And that was all from driving posture and smoothing out his wheel inputs.