One very passive method that has worked well for me in the past was to prepare a highly-concentrated salt solution (5M -- 292 g salt per liter of water) and then let the water evaporate very slowly (like only slightly open to the outside world) and left undisturbed. This left me with a roughly 2cm x 2cm x 1cm crystal of salt that is incredibly transparent (save small impurities in the center) with nice cubic features on the broad face.