One mistake I've made over the years, with a few exceptions, was to NOT keep a copy of a completed project. I've been in this profession 19 years; I've done alot.
Recently I was asked to provide work samples to a prospective employer. (Fair enough - I've asked the same of people I've interviewed).
I realized I had to do 2 things:
1) Rely on the internet, where possible, to download and either burn to CD or bind on pretty paper. Since we will be reviewing the work samples over lunch, I've spent the weekend re-creating documents I can find on line from client web sites.
NOTE: I was disgusted with one past employer: Documents that Prudence and I created had been re-badged under the credit of CURRENT staff. How unethical is that?
2) Purchasing hard copy as a public open record.
SO THE MORAL OF THE STORY:
Always Always Always keep a copy of your work!