Tips for maximizing yield, purity and crystal size during recrystallization
When I grew crystals of aluminium potassium sulphate I filled a jar with boiling water and placed it in a cooking pot that was also full of water. Doing this helps to maintain a constant temperature. I set the cooker to a low heat and began dissolving the alum until the solution was saturated. At this point I removed the jar containing the solution and suspended a piece of string for a crystal to grow on. This would be my seed crystal. I left the jar on a window sill to cool and waited.

A few days later a crystal had formed so I lifted out the string and set it aside. Then I reheated the solution to dissolve any crystals that had formed on the jar as they would compete with the suspended seed crystal, reducing its size. It is also a good idea to filter the solution at this point. Once I had finished this step I placed the string with the seed crystal back into the jar and placed it on the window sill again. After a week I moved the jar to a refrigerator (around 3 degrees C)  to further cool the solution. This left me with a pretty big, well shaped crystal.

I didn't do this but you could try evaporating off the water after the previous step to grow the crystal further.