A great way to keep your stencils organized is to get a package of page protectors and a 3-ring binder. I put paper in mine so I can use both sides of the protector and the stencils won't get tangled. For slightly larger sized ones I cut the side of the protector open. When there are alot of little ones I clip the protector closed with a paper clip. After alot of use your stencils may be tacky so keeping them stored this way helps avoid a sticky mess.
I find there are some terrific commercial stencils available at craft stores but they aren't usable AS IS. For instance, this Fleur-de-lis stencils....I would not lay it down on a page and spray. You could mask off the unwanted images with scratch paper, or you could alter them as seen here with the butterfly. This was from a full collection of butterflies. I cut it apart, than attached it, with packing tape, to a blank sheet of acetate to isolate the image as it's own stencil. Don't forget to cut an opening in the acetate that roughly matches the outline of the image.
Once you've been using your stencils over and over they become a work of art themselves. If you invest in the cardboard letter stencils, consider getting several sets as eventually you may want to use the actual stencil IN your art. You can see I love to collect alphabet stencils. Speaking of collecting.....we have started the next GPP Street Team Crusade. Crusade No. 6: Pick Your Poison, is about journaling the things you collect. Congrats to Juliet Wilson for being the Top Hat Draw for Crusade No. 5. Don't know what I'm talking about? Come see over HERE.
P.S. Don't forget to visit my pal Lisa's blog. She's become a devoted blogger and this week she covered more about stencils.