From Snowflakes and Thread Crochet's Crocheted Snowflakes Stiffeners page
When I've crocheted beads into my snowflakes, or used metallic threads to make them glitter, I prefer to stiffen them with starch. I use plain old cornstarch, since I can't find non-spray laundry starch around here anymore (and the spray stuff just isn't strong enough). Simmer a tablespoon of cornstarch in 1/2 cup of water until it becomes transparent. Be careful if you want to simmer it in the microwave because it likes to boil over and somehow slime the entire interior surface of the oven of the oven. This makes a huge mess. Believe me, really, I've been there, done that (repeatedly. I'm stubborn). I think it's better to nuke a small amount of starch gloop in the bottom of big quart measuring cup, cooking on high and watching it every moment. Stop and stir every minute, and take it out as soon as it thickens up. If you don't have a big cup, use a big covered dish, and don't let it boil over! Do be aware that freshly cooked starch slime is very hot, and it sticks to your skin much better than you'd like it to. Treat it with respect.
Keep your cooked starch in the fridge between uses. As with the glue stiffener above, I keep it in a wide mouthed half-pint deli container, convenient for dipping snowflakes, but with a lid to put on when I'm done. I've heard that a little essential oil of mint or cinnamon is supposed to keep things from growing in it, but nothing has grown in mine yet, so I haven't bothered.
To starch your snowflakes, just let them soak it up, blot them off with a paper towel, then block them.
For larger amount of starch stiffener, from How to Stiffen Crochet on eHow:
Add 1/2 cup of cornstarch and 1-1/2 cups cold water to a heavy saucepan. Stir until the solution is smooth with no lumps. Cook on medium-low heat, stir constantly until all of the cornstarch is dissolved and the solution is thick and transparent. Remove from the heat and allow the liquid starch to cool to room temperature. Dip the crochet item into the starch. Squeeze out the excess liquid gently without wringing or twisting. Shape according to pattern instructions or as you usually do and allow it to air dry completely. This non-permanent stiffener rinses out with water so it's best for pieces that require occasional laundering, such as doilies. Re-stiffen the item after laundering.
Both of the above sites suggest 50/50 mixture of (white generic (eg Elmer's) glue and water for stiffening