The easiest way to change the evaporation rate is to change the temperature. Hotter water evaporates much faster. It's a little tricky to do this in a way that doesn't also change how rapidly the solution stirs, which also can affect crystal growth. I can think of two suggestions:
1. For both a warm and a cold solution, use magnetic stirrers so they're well-stirred.
2. Make sure that both the warm and the cold solution are maintained in large stable baths of fixed temperature, so you don't get the sort of mixing caused by temperature differences between parts of the solution.
Of course, temperature can also affect the crystallization directly, so that complicates this experiment. Perhaps another way to do it would be to make the container nearly identical, kep them at the same temperature, but put a partial cover on one. That would slow down its evaporation.
(published on 08/27/2010)