The music production process usually produces a minimum of 3 sets of master recordings:
Multitrack Masters – the complete, edited multitrack recordings that fed the mixing process, e.g. Pro Tools sessions, 2-inch 24-track tapes, etc.;
Mix Masters – the various 2-tracks and bounces that fed the mastering process, e.g. stereo audio files, ½-inch 2-tracks, etc.;
Replication/Distribution Masters – the media that came out of the mastering process to facilitate distribution, e.g. CD replication master(s), file sets for upload, etc.
My project/session files from early 2000s are no longer compatible with my current DAW setup. Mostly due to the fact that I've changed my primary DAWs multiple times and I switched to Mac. In addition, plug-ins I have installed on my system that I use these days are very different to what I used to use in 2005. And now, all new cool iPad DAW apps give us even more project file types.
Since there is no universal session file format and project files (and their assets) are binary, the best way to archive your sessions files is to bounce down individual tracks (pre and post channel strip) store them along with your mix masters. Basically what Schlette says with one addition, I would archive important MIDI files too.