I buy frames from the craft stores or good will. I mat the portrait and frame in the next biggest size. I back my portraits with felt.
Depending on how the cardboard backer attaches to the frame, you can force everything back together. if the cardboard has the tabs that rotate into a grove, reversing the cardboard and using a pick to lock the tabs in place. (that sounds way complicated, I will try to get some pictures sometime soon)
I get all of the layers as secure as I can, then lock them in place with a few generous blobs of hot glue on each side.
The last thing I do when framing pieces is to cover the back side with kraft paper. I use hot glue to seal it around all 4 sides. It makes the framing job look more professional and hides any ugly things I have done.
I have a project that is ready for a frame, I will take some pictures to explain my process, hopefully before the weekend
Katie