Making the words un-breakable may lead to underfull or overfull boxes. I would say that the best solution is to make the section headers \raggedright, which ensures that they break only at spaces and no bad boxes are introduced. As well, it looks better than the hyphenates section titles IMO.
Recommanded solution with titlesec package. Put this code into your preamble:
\usepackage[raggedright]{titlesec}
Solution with no package. This redefines the inner section commands to use \raggedright. Put this code into your preamble:
\makeatletter
\def\section{\@startsection{section}{1}{\z@}{-3.5ex \@plus -1ex \@minus -.2ex}{2.3ex \@plus .2ex}
{\normalfont\raggedright\Large\bfseries}}
\def\subseciton{\@startsection{subsection}{2}{\z@}{-3.25ex\@plus -1ex \@minus -.2ex}{1.5ex \@plus .2ex}
{\normalfont\raggedright\large\bfseries}}
\def\subsubsection{\@startsection{subsubsection}{3}{\z@}{-3.25ex\@plus -1ex \@minus -.2ex}{1.5ex \@plus .2ex}
{\normalfont\raggedright\normalsize\bfseries}}
\makeatother
\mbox{model}. – Werner Aug 21 '12 at 23:52