How to force output to a left (or right) page?

For odd / right-side pages you can use \cleardoublepage in two-sided documents. (In one-sided ones \cleardoublepage is identical to \clearpage)

To force even / left-side pages you would need to define your own macro based on \cleardoublepage, but with reversed logic:

\documentclass{book}

\newcommand*\cleartoleftpage{%
  \clearpage
  \ifodd\value{page}\hbox{}\newpage\fi
}

% Demonstration / Test:
\begin{document}
  Title on right side
\cleardoublepage
  other title on right side
\cleartoleftpage
  on the left side
\cleartoleftpage
  also on the left side
\cleardoublepage
  right side again
\end{document}

A more complex version which behaves nicely in single-sided documents and also supports two-column mode is:

\makeatletter
\newcommand*{\cleartoleftpage}{%
  \clearpage
    \if@twoside
    \ifodd\c@page
      \hbox{}\newpage
      \if@twocolumn
        \hbox{}\newpage
      \fi
    \fi
  \fi
}
\makeatother

This is the definition of \cleardoublepage just with the \else removed to negate the logic, i.e. make it force even pages instead of odd.

As Martin explained, \cleardoublepage is a standard LaTeX command to force a break to an odd (right, recto) page in double-sided documents. The KOMA-script classes and the memoir class offer numerous custom commands for page-breaking:

• With KOMA-script, use \cleardoubleevenemptypage to force a break to an even (left, verso) page. (If necessary, this will produce an additional odd page with page style empty, as is the default in KOMA-script.) See section 3.13 of the KOMA-script manual for other available commands.

• With memoir, use \cleartoverso to force a break to an even page. See section 18.13 of the memoir manual for other available commands.

You can modify both \cleardoublepage (as in this answer) and @Martin Scharrer's code to insert, if necessary, an empty page:

\makeatletter
\def\cleardoublepage{\clearpage\if@twoside
\ifodd\c@page
\else\hbox{}\thispagestyle{empty}\newpage
\if@twocolumn\hbox{}\newpage\fi\fi\fi}
\makeatother

and

\makeatletter
\newcommand*{\cleartoleftpage}{%
  \clearpage
    \if@twoside
    \ifodd\c@page
      \hbox{}\thispagestyle{empty}\newpage
      \if@twocolumn
        \hbox{}\newpage
      \fi
    \fi
  \fi
}
\makeatother