I often need to write various thing in subscript and superscript which have nothing to do with maths so I don't want to have them written in math mode. For LaTeX users there is no easy way to write subscript and superscript in text mode.
XeTeX, however, has prepared some macros for this. Assuming you have \usepackage{xltxtra} in the preamble, you can use the commands

\textsubscript{}
\textsuperscript{}

if you want something like C_2 you write C\textsubscript{2}. Of course if you use this a lot, it is much more convenient two write some shorthand command to use instead these long ones.

Those of you who still write in LaTeX may find a solution here.