Space between lines

Insert \par inside the {\LARGE...} group and remove the line break at the end, since "There's no line here to end." Or, just drop the group formatting entirely, letting the font selection be scoped by the center environment:

\documentclass{article}
\begin{document}

\begin{center}
{\LARGE\bfseries An \textit{a priori} Typology of Sentential Negation from an HPSG Perspective}
\end{center}

\begin{center}
\LARGE\bfseries An \textit{a priori} Typology of Sentential Negation from an HPSG Perspective
\end{center}

\end{document}​

The motivation here is that TeX only sets a paragraph once it's fully read it. So, without a blank line or an indication like \par the paragraph isn't set correctly with the change in \baselineskip.

Please note that the \it, \bf, etc. font macros are deprecated because they do not use the new font selection scheme introduced with LaTeX2e. Please use {\itshape ..}, {\bfseries ..} or \textit{..}, \textbf{..} instead. See Does it matter if I use \textit or \it, \bfseries or \bf, etc. and Will two-letter font style commands (\bf, \it, …) ever be resurrected in LaTeX? for more information.

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With your updated post, here are some things to try, using the concepts described above:

\documentclass[11pt,a4paper,fleqn,draft]{article}
\usepackage{url}% http://ctan.org/pkg/url
\begin{document}

\begin{center}
  {\LARGE\bf An \textit{a priori} Typology of Sentential Negation from\par an HPSG Perspective\par}

  \bigskip

  {\large Joshua Crowgey\par}
  University of Washington

  \vspace*{3\bigskipamount}

  Proceedings of the HPSG 2012 Conference

  \bigskip

  Department of Linguistics, Chungnam National University Daejeon, South Korea

  \medskip

  Stefan M{\"u}ller (Editor)

  \medskip

  2012

  \medskip

  CSLI Publications

  \medskip

  \url{http://csli-publications.stanford.edu/}
\end{center}

\newpage

\end{document}​

1. Add \usepackage{setspace}.
2. To change line spacing for whole document use \setstretch{baselinestretch}. For some custom text use environment: \begin{spacing}{2.5} ... \end{spacing}.

That's, I think, what's you need.

It depends on the document class you are using but you can also use the \title{}. And see Does it matter if I use \textit or \it, \bfseries or \bf, etc

\documentclass{article}
\title{\textbf{\LARGE An \emph{a priori} Typology of Sentential Negation from an HPSG Perspective}}
\date{}
\begin{document}
\maketitle

\vspace{-3\baselineskip} % If you wish to remove the space for date and author
Start of the text part .... 
\end{document}