You can use \linewidth to specify the width of the line so that if the \linewidth does change this won't need adjustment, as I have done for the first rule in the example below. This will also adapt to the column width if you use more than one column.
If you want to get fancier, there are many options as to what to use as a separator. To illustrate, I have used an ornament from this question and another from this question on how to insert a border:

\documentclass{article}
\usepackage{xcolor}
\usepackage{lipsum}
\newcommand*{\MyLine}[1][black]{\par\noindent{\color{#1}{\underline{\hspace{\linewidth}}}\newline\par}}
%------------------------
% Ornament from https://tex.stackexchange.com/questions/11320/end-of-paragraph-with-ornament
\usepackage{pifont,fourier-orns}% These are needed only for the ornament
\newcommand\crulefill[1][1ex]{\leavevmode\leaders\hrule depth \dimexpr-#1+0.4pt height #1\hfill\kern0pt}
\newcommand\ornline[2][1ex]{\trivlist\item\crulefill[#1]#2\crulefill[#1]\endtrivlist}
%------------------------
%https://tex.stackexchange.com/questions/30973/how-do-i-insert-a-border-below-text/30979#30979
\usepackage{xcolor}% http://ctan.org/pkg/xcolor
\usepackage{xparse}% http://ctan.org/pkg/xparse
\NewDocumentCommand{\myrule}{O{1pt} O{3pt} O{black}}{%
\par\nobreak % don't break a page here
\kern\the\prevdepth % don't take into account the depth of the preceding line
\kern#2 % space before the rule
{\color{#3}\hrule height #1 width\hsize} % the rule
\kern#2 % space after the rule
\nointerlineskip % no additional space after the rule
}
%------------------------
\begin{document}
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Etiam in erat vitae libero dictum imperdiet volutpat nec orci. Nam vel magna vitae risus tristique ornare. Etiam iaculis tincidunt ante, sed consectetur dui sagittis at.
\MyLine[red]
Duis porttitor lacus ut sapien varius ut dapibus odio dapibus. Pellentesque sed odio et nisl lacinia rutrum at quis diam. Mauris a nulla sed est blandit varius eget id massa.
\ornline[0.6ex]{\decoone}
Vivamus libero nibh, dignissim laoreet sollicitudin nec, lobortis nec orci. Pellentesque dolor libero, pretium ut euismod nec, tempor sit amet diam. Mauris vestibulum ornare lacus, in tincidunt dolor adipiscing vel.
\myrule[5pt][5pt][orange]
Maecenas mattis sodales justo, ut fringilla leo vulputate sit amet.Vivamus in felis quis justo viverra blandit id mollis velit. Aenean mi dolor, pulvinar eu ornare et, hendrerit sed tortor. In vitae vulputate felis.
\end{document}
\sectionof your document? – Gonzalo Medina Dec 16 '11 at 17:55