Okay, maybe it's not a secret, but I've never seen anyone else use this idea before. My favorite design secret is to decorate with scrapbook paper, but only the square kinds (size 6x6 or 12x12 inch) and put them in frames.
I place the frame on my mantel and switch out the scrapbook paper all year round. I have Halloween paper, Christmas paper, Fall paper, Spring paper, etc. And then I also have some of my favorite designs that I display at other times.
In our baby's nursery, I have smaller 6 x 6 frames with prints in them. I love how easily I can switch out the paper!
My 12 x 12 white frame I got from Target in-store. Here's a link to a similar one online: here
And I got the 6 x 6 white frames on Amazon: here