I've been making decals for a little while now and ones I have made months ago are still sticking to my windows. You have to use the smooth side (the side that was touching the build plate) and make sure the surface you are sticking it to is clean. Wet with water and place on the window, mirror or shower wall. Overall the lower the shore hardness of the flexible filament you use NinjaFlex = 85A the better they stick. I've found that my new house brand Flexy sticks nicely as well even though it's 96A.