Make a super-saturated solution of Borax and water:
--Fill a jar with hot water (boiling is best).
--Add Borax, a little at a time, until no more will dissolve (you'll know you're there because instead of dissolving the Borax will settle to the bottom)
Use pipe cleaners and thread to make a snowflake.
Attach a piece of thread to the snowflake.
Place the snowflake in the Borax solution and leave for several hours or overnight.
In the morning, you'll have a beautiful, sparkling snowflake, covered with large crystals.
If you'd rather not make snowflake shapes, you can shape the pipe cleaner into stars or other shapes. You could also just place a straight pipe cleaner into the solution.
The pipe cleaner works well because all the fuzz on it gives the crystals nice places to attach, and thus works much better than just a string. (Which may explain why all my attempts at making rock candy as a kid were met with utter failure (and a sticky mess)).
Safety Note: The Borax and the finished snowflake should come nowhere near the mouth.