I was looking through some old files recently, and it occurred to me that they might be obsolete in the future. Are there any steps that I should take for "archiving" my documents so that they will compile properly on later versions of LaTeX? I once went to a seminar where it was indicated that the .dvi file can be converted back into a plain TeX file, so would it be the best "archival" document?
Another package which helps is snapshot. It provides a list of all packages that are used by a document, so you can package with (or arrange to keep in your system the right versions of) everything needed to reproduce your archived document.
Plain Tex source files that use only Computer Modern fonts, per Sharpie's comment, is the most change-resistant Tex format there is.
I once went to a seminar where it was indicated that the .dvi file can be converted back into a plain TeX file
It can be true, if specials are inserted into the DVI, but by default it is not true, and the meaning of inserted specials is left wide open to interpretation: are they meta-data or Postscript? Special-less DVI is a well-documented, equally change-resistant format, but a problem here is that the DVI file stores information about fonts in a way that is open to interpretation. If you only use Computer Modern fonts, this won't be a problem.
Generally speaking, Patrick's answer is right: archive your Tex installation.
An option is to use a Virtual PC to insure the same exact environment is kept for future use.
Download VBox (free). Create a Virtual PC, install Linux (or windows) but I'd go with Linux since it is free and easier to work with for Latex, and install the tex distribution which you used/ are using and anything else needed to generate those documents with.
When done, then backup this Virtual PC (it is one file) to a DVD or CDROM or another backup USB disk, and save it locked somewhere safe. You can make as many virtual PC's as you want. Make them small enough just for Latex. Each is just one file.
Now anytime you want to reproduce the old documents, you have the full Virtual PC with everything in it to do so. All the environment, user settings, user account, everything, since it is the whole Virtual machine which is saved. Boot the Virtual PC and it is all there. This is the only way to guarantee that the same setup is used next time you want to run Latex on these file again.
I restored .tex documents from 20 years past, without problems. But i wasn't that adventurous then, so it isn't an issue. But it may be an issue in future. I used to do the stuff using an ancient version of The Semware Editor under OS/2's Emtex environment. I now use WinEdt 6.0 /LiveTex under Windows 2000.
TeX and LaTeX already do deal with multiple encodings, so if you migrate from one environment to another (eg OS/2 to Linux), then Latex will still recognise that the files are written in a different coding and handle these accordingly.
My approach is to do things to make it easier to back up. Specifically, packages to install are kept in a running archive, and documents and files are kept in a tight bundle. The idea is that one might back up the bundle and running archive, and join this to an installation of ghostscript / TexLive etc.