I'm sharing one of my super-secret high-tech collage tools today...even though you probably already do this!
I frequently have trouble "seeing" what something will look like when it's all put together, or it looks really different to me after everything is glued down.
To help me see the final result, I grab a piece of cardboard or cardstock & cut a hole in it, the same size of my final collage.
I have one of these for ATCs; the opening measures 2 1/2" X 3 1/2". There's another for my gluebook, which is a pocket moleskine sketchbook.
When I play with papers, I lay the frame over the top to see what the final product will look like. It helps me focus on the final look of the piece without the distraction of the parts I might cut away.
Super easy and really helpful.