\documentclass{article}
\usepackage{psfrag}
\usepackage[latex={-interaction=nonstopmode},crop=off]{auto-pst-pdf}
\begin{document}
psfrag example with replacements defined in example-psfrag.tex:\par
\fbox{\psfragfig{example}}\par
\bigskip
the same example with a psfrag override (bottom left):\par
\fbox{\psfragfig{example}[\psfrag{[Mp]}{$N_Q*$}]}
\end{document}