sodium hydroxide increases the conductivity of the solution, thus this increases the efficiency of the electrolysis, but salt bridges are always a problem, because they aren't 100% efficient, you'll have problems with the formation of junction potentials, so that's another way to improve the efficiency-by considering better salt bridges. Catalysts don't "use up the voltage" so I'm not quite sure what you're referring to here.
They are actually many ways and factors to consider, some of these factors are interrelated so you'll need to find the optimum conditions and voltage. The reason they say to lower the voltage stems from the polarization effect with high voltages.
I'll try looking into this a bit more later, at the moment I'm a bit preoccupied with something else. You can keep asking general questions, but as far as specific methods go, I don't have one in mind at the moment; also be more specific and detailed about what you wish to achieve.