Having seen how several of you model your 'heads' by the half, then copy mirror it to create an exact opposite side of the face, what is the command to copy, mirror an object?

Is this way the recommended method of modelling heads? As I have been modelling the head as a whole which has become a long pain staking process to say the least to get it looking remotely even/ symmetrical.

Also, are you able to mirror multiple objects at the same time?
I have constructed pretty much a futuristic mercenary and would like to mirror 'his' arm and leg across as it was quite in depth (well, for me it was) session/s of modelling to get them how I intended.

Thanks all.