I don't know if what I do would really qualify as a portfolio, it's more of a photo diary of my cardmaking. I make a photocopy of every card I send out and make a note of when it was sent, to whom it was sent and for what occasion, I will also photocopy what greeting was on the inside (I make my own "insides" on the computer so that they are like store bought cards.) I put the photocopy of the card in sheet protector (actually, I usually have 2 cards on each photocopy and 2 sheets in the protector back to back) and then it goes into a 3 ring binder that is dated by year.
This has been a really valuable tool for me because I am able to look back according to the month/year and see what card I sent to someone for their birthday and what the inside greeting was. It also is an accurate record of cards that I've made and if I want to recreate the card I can go to the notebook and figure out the colors, papers and techniques that I used.
Hope you get some more ideas!