Oh how cool! Goodluck! Here's a website on various ways to stiffen the flakes after they are pinned. This gal is really awesome and she has tons of snowflake patterns on her site as well. I tried a few more products myself and I love the "Stiffy" fabric stiffener the best. Since this is the first time I've used it I don't know about yellowing over time but it dries super stiff in just a couple of hours under a fan, with only 1 application. The Elmer's school glue, watered down, also works well (and from what I've read doesn't yellow badly) and it's non-toxic but it took over 24 hours for it to dry thoroughly. It is much cheaper though! lol Apply either with a 1" stiff paint brush.
Oh I almost forgot. The glue holds the glitter on better but you probably know that already.