I always do it straight forward but there might be a function for this; group all of it (taken you've started modelling from one axis to the left or right (fwd or back, just how you set it up...)) and then simply duplicate and negative scale the duplicat. 
If you've moved the entire model to god knows where use a locator as a parent instead.
Cheers, Maos.
"there are no stupid questions just stupid people" (Dinkey)