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A rounded nose that gradually widens will help maintain the laminar boundary layer. The laminar boundary layer will
Reducing the surface area will decrease drag, but may not be feasible due to neading surface area for a solar panel
A smooth surface is the most important factor in keeping a a laminar boundary layer.
A thinner body is desirable as well, because the reduced stagnation pressure at the rear of the car, and therefore the resultant pressure difference between front and back, will be acting on a larger area for a wider car.
Avoid abrupt angle changes along the body
Design to generate close to zero lift.
Note that these design ideas often contradict. If you want to maintain a solar array area but reduce thickness, the car must get longer which will increase the length that the skin friction acts on. It is about balancing these conflicting interests in the best way possible.