My strategy changed, though, with the arrival of cloud storage. Now I have all of my files—about a terabyte and a half—synced to multiple computers through Dropbox, and I back up each of those computers to an offline hard disk every ten days or so. My main computer is also backed up continuously to an external hard disk.
File formats are a different problem. Until around the end of the 1990s, I used a series of different word processor, spreadsheet, page layout, email, and other programs, all for the Macintosh, and I can no longer open a lot of those files. In more recent years, file formats seem to have become more stable, and I have tried to be careful to use formats that seem likely to survive as long as I will—docx, odt, mp3, pdf, jpeg, and especially txt.