In the modern age, it's really easy.
Make sure all your stuff is electronic.
Then just keep all your emails.
I signed my last lease agreement, my mortgage, my divorce, etc. all electronically. Employment contracts - If they aren't already electronic, I scan them in and email to myself.
My email account goes back to about 2000. I have the email accounts before that in a file somewhere (it's only about 1Gb). In work, I have policies that basically all result in "if you want this, file a ticket, fill out a form, put it in an email" and then helpdesk tracking of everything and EVERY EMAIL ever sent or received from my account since I started there.
It's really not that hard, in the modern era.