In addition to holding them in your heart, it would be great to write your promises in a journal like a recent client. I loved her little notebook that she clung to. It was pretty, and special and she looked beautiful reading from it - as opposed to a crumpled paper.
I can only imagine how many times one might reflect on their vows. I am reminded at every wedding I am honored to attend. It would be lovely to have a keepsake.
I found all these on Etsy: