How to use mirror so that you model only one side of the mesh and then just mirror it on other?
For example, in modeling this dragon I would like to model only one side and then mirror it to other.

Personal opinion ... don't use virtual mirror. Just use loop-cut and mirror only as often as you really need it.
I would to what Micheus did ... except I would pick face/mirror rather than virtual mirror.
I don't like the constant updates while I'm working. I only need it every so often.
I use virtual mirror while modelling in order to be able to get the best view angle, however, in the end i remove the virtual one and i pick a face and mirror it in the end.