Select th eedges you want to be smooth and choose EditPolygons>Normals>Soft/Hard options and set to Smooth. Select all the edges you want to be hard and do the same thing, only making them hard with the Soft/Hard normal command.

edit* -or if you actually want to perform the smooth operation on specific parts of your model, it's just a matter of selecting the faces you want to smooth, then using the Polygons>Smooth command.