\documentclass[12pt]{article} 
\usepackage{graphicx,psfrag}
\begin{document}
%{\bf A simple figure}
\psfrag{a}{$\pi$}
\psfrag{2a}{$2\pi$}
\psfrag{-2a}{$-2\pi$}
\begin{figure}
\includegraphics[height=.5\textheight,width=.7\textwidth]{plot1.ps}
\end{figure}
\end{document}