What method is used to separate ammonium chloride and common salt?

There are several processes to separate mixtures like : 
 A) Evaporation 
 B) Filtration
 C) Sublimation 
Ammonium chloride is separated from common salt by the process of sublimation. 
Common salt does not decompose into evaporation hence the sublimation process is used. Sublimation is the process where substance is transferred from solid state to liquid state without passing thorough liquid state.
Solid The mixture of ammonium chloride and common salt is taken in to china dish. Keep the china dish on a wire gauze which is placed on tripod stand and cover the china dish with an inverted glass funnel and place in a little cotton at the opening of the stem of the funnel. Vapors of NH4Cl got condensed at the tail end of funnel and get separated. Common salt remains at the bottom of china dish. 
 
 

The easiest way to separate common salt and ammonium chloride from a mixture is through the process of sublimation. 
Sublimation is the process in which a solid substance is directly converted to the gaseous phase, without the intermediate liquid phase. Camphor is a common example of a substance that undergoes sublimation. Similarly, ammonium chloride undergoes sublimation. When ammonium chloride is heated, it gets converted into ammonia and hydrochloric acid. The chemical reaction of this process can be written as:
NH_4Cl (s) -> NH_3 (g) + HCl (g)
Common salt (also known as sodium chloride), on the other hand, does not undergo sublimation. 
Thus, if we heat a mixture of ammonium chloride and common salt, the former will convert to the gaseous phase, while the common salt will be left over as solid. The gases can be collected and cooled to obtain ammonium chloride; thus obtaining the two salts separately. 
 
Hope this helps