How can a designer maximize the glide ratio of an aircraft with low aspect ratio wings, other than by increasing the aspect ratio?
With a given wing span, everything that reduces drag will increase the gliding range. Aspect ratio is less of a factor here because induced drag depends on wing span, not aspect ratio. But increasing the aspect ratio with a given wing span will reduce the wetted surface and, consequently, friction drag.

What can be done to increase gliding range outside of increasing span?


retract the landing gear,
avoid struts or bracing wires,
minimize friction drag by allowing for more laminar flow and minimum surface area. 
minimize pressure drag by streamlining everything


Generally, a low aspect ratio shifts the polar point of best gliding range to lower lift coefficients, so reducing all that parasitic drag becomes more important the stubbier the wings are.

The most effective way, however, to increase gliding range is indeed a larger wing span if all other parameters are held constant. Increasing span while keeping wing area constant will:


increase aspect ratio and reduce wing chord,
reduce induced drag at the same lift coefficient,
and, as a consequence, lower the optimum gliding speed, so the parasitic drag is lower as well.