buk, the solvent should be super-saturated for nice xtals to grow, if your solvent isnt super-saturated the xtals will have a hard time grow, and they will just end up being clumps of spice.
Personaly when I want to get those big xtals, I do my first freeze precip, then I collect the Spice(clean it if needed), then dissolve it in a small amount of solvent, then I cover the evap dish with a coaster and just leave a small gap for the solvent to evap. Leave it somwhere where it wont be disturbed for a few days. Or you can go with soulfood's way. Experiment a bit and find what way you like best.