|See also Let's Solve the File Format Problem wiki that provides an extensive catalogue of file formats.|
A very good rule of thumb with data formats is to pick those that are no more complex than the data being represented, that are recoverable with simple tools and widely implemented.
In general, if you have written a text document and it's not viewable or editable in a low-level text editor (Notepad, Emacs, and so on), you should probably take the time to convert it into a plain-text format - keep the rich format also.
If you are backing up data in a format that's not widely understood, be sure to also keep backups of the software you use to open it and any registration keys. A file made with version 2.x of a piece of software may not open with the all new, singing and dancing version 5.x!
Plain text, HTML and non-bloated XML formats are all good bets (DocBook, TEI etc.).
The Portable Document Format standard created by Adobe has reached a point where it should be readable for posterity. The format is now open enough that it should be usable for backup for the forseeable future; PDF/A is specifically designed for digital preservation. You can get a PDF reader here.
The TeX standard has been around since 1969. TeX documents are text based. It is widely used to prepare multi-thousand page documents for publication, as well as mathematical formulas. LaTeX is an free, open implementation of this standard.