Define the margin parameters:
\documentclass[]{article}
\setlength{\marginparwidth}{1in}
\let\oldmarginpar\marginpar
\renewcommand\marginpar[1]{\-\oldmarginpar[\raggedleft\footnotesize #1]
{\raggedright\footnotesize #1}}
then add a note
\marginpar{note: this is an important section}
Source: http://www.f.kth.se/~ante/latex.php