I generally use the following preservation system:

(1)  Gluconolactone/Sodium Benzoate (1.25%)

(2)  Trisodium Ethylenediamine Disuccinate (0.3%)(chelating agent) + Citric Acid (0.2%)(Preservative Booster)

(3)  1,3-Propanediol 3.0% (Preservative Booster)

Add all of the above to your water phase right up front under high shear, then heat to 77C and hold for 20 minutes.  That generally ensures the Gluconolactone/sodium benzoate stays in solution. 

Based on some research that I came across recently, I am going to swap the Trisodium Ethylene … and replace it with Dissolvine GL-47-S (Tetrasodium Glutamate Diacetate) … the studies showed a significant enhancement in preservative efficacy using Dissolvine + Citric Acid.