Environment decorators

Have a look at the etoolbox package, in particular to its \AtBeginEnvironment command. This allows you to add aditional code to the begin of environment.

\documentclass{book}

\usepackage{etoolbox}
\AtBeginEnvironment{quote}{\hrule}

\begin{document}

\begin{quote}
test
\end{quote}

\end{document}

The LaTeX kernel provides \g@addto@macro which works for the environment starter command as well, i.e. for the environment quote the start macro is \quote, so say \g@addto@macro\quote{foo} etc.

No extra packages are needed for this, apart from the specific additions that are to be made, but that depends on personal choices of the O.P, but in general 'anything' can be placed in \g@addto@macro\quote{...}.

\let\oldquote\quote
\renewcommand{\quote}{fooaddition\oldquote}

would work as well, but this may fail for other environments that have optional arguments. In this case, \LetLtxMacro from letltxmacro package is needed, which means another package, however.

\documentclass{book}

\makeatletter
\g@addto@macro\quote{\hrule
  \medskip

  \textit{be careful}

  \medskip

  Now the real stuff begins\dots

}
\makeatother    

\begin{document}

\begin{quote}
test
\end{quote}

\end{document}