Position a super magnet on the inside of you hat where you want the bling to attach on the outside.
Snap another super magent onto the outside of the hat.
Attach anything metal to the outer magnet. These are the things I used here: a broken necklace, a pair of scissors, a handfull of paper clips, a Christmas tree ornament, and the inner guts of my steampluck lamp
Not shown here, I also used a lid from a soup can (a Warhol inspiration).
Optional: hot glue the outer magnet to anything non-magnetic, and you're increased your options exponentially.