If you don't want to apply using eyelets in the corners or with something else like photo corners (or other corner type embellishments), run it through a Xyron machine or similar that applies a solid, clear adhesive evenly across the entire back side. Make sure to smooth it using a bone folder, and do that with a nice even surface behind the material you're working on (ie, don't smooth it on a wooden table or work surface that has any texture).
Hope that makes sense. =)