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.
Most of my oldest documents were Wordperfect 5, so they were still readable. I have converted them to txt (accessibilty) and pdf (preserve formatting).
My biggest miscalculations were using CD-RW media during a period of time in the early 2000s (the media often cannot be read by CD drives the disc was not created on) as well as my choice to use cheap CD-R media which over time the plastic became cloudy, or the reflective layer oxidizing and then flaking over time.