How to slice smoked salmon really thinly?
First off, chill it in the refrigerator overnight.  Preferably in the coldest part of the refrigerator (the bottom).   After that put it in the freezer just long enough to get stiff enough to slice thinly, but do not let it completely freeze.   

The initial chill will help keep any ice crystals that form inside the cells small, thus decreasing the risk of the meat getting mushy.