we can check a trans losses by open circuit and short 
sircuit test.
to minimise these losses we shuold have a good cooling 
system. apart from that all other points are related with 
the insulation of the core,permitivity etc.

we can check transformer losses by short ckt and open ckt test
,for checking cu loss we use short ckt test and for iron
loss we use open ckt test..to reduce copper loss we should
go for good cooling and for iron loss the core should be
perfectly laminated

transformer losses can be found by doing open circuit test 
and short circuit test
open circuit test gives the core losses
short circuit test gives the copper losses
note in open circuit test the lv side is given supply and 
in short circuit test the hv side is given supply

the iron losses can be reduced by providing laminations of 
thinner dimension and using c.r.g.o sheet for laminations
 
the copper losses depend on the winding resistance which 
increases due to heat can be reduced by providing 
sufficient cooling to the winding.