I have kept my QF and AN boarding passes and e-tickets, mainly since I joined FF a few years ago. I also have everything from my international travels.
I have a quite a few [precious] Ansett ones, including my last Ansett A320 flight on 29 May 2001.
The domestic flight ones I keep in a folder with other receipts and things, and the international ones are in a separate book.
From Australia. Qantas - Spirit of Australia.