I use the following command to have section headings with subtitles
\newcommand{\Section}[2]{\section[#1]{#1\\\small »#2«}}
But now I only can have numbered sections because the * doesn't work for my own command. How can I have my command working both ways, with and without the asterisks?
This is the standard way to define *-variants:
\makeatletter
\newcommand{\xyz}{\@ifstar\@xyzstar\@xyz}
\newcommand{\@xyzstar}<what to do when there's a *>
\newcommand{\@xyz}<what to do when there's not a *>
\makeatother
In you case it's probably
\makeatletter
\newcommand{\Section}{\@ifstar\@Sectionstar\@Section}
\newcommand{\@Sectionstar}[2]{\section*{#1\\\small »#2«}}
\newcommand{\@Section}[2]{\section[#1]{#1\\\small »#2«}}
\makeatother
The xparse package provides an intuitive means for incorporating stars * in commands. Conditioning on whether you are using a starred/unstarred version is done using \IfBooleanTF:
\documentclass{article}
\usepackage[T1]{fontenc}% http://ctan.org/pkg/fontenc
\usepackage{xparse}% http://ctan.org/pkg/xparse
\NewDocumentCommand{\Section}{s m m}{
\IfBooleanTF{#1}%
{\section*{#2\\\small \guillemotright #3\guillemotleft}}% Unnumbered section
{\section[#2]{#2\\\small \guillemotright #3\guillemotleft}}% Numbered section
}
\begin{document}
\Section{First section}{first title}
\Section*{Second section}{second title}
\end{document}
xparse is easy to read in terms of the code, rather than following a sequence of commands. In my answer, xparse is an additional package that has to be loaded (that comes with additional functionality). In @egreg's answer, manual definition of 2 commands is required (one for the starred * and one for the unstarred version). your choice.
It might be preferable to control section numbering globally. That is, you use non-asterisk sectioning commands everywhere in your document and then you use a command to say which section levels will be numbered.
Here's how to do it. If you place
\setcounter{secnumdepth}{1}
in your preamble, then only the highest level section will be numbered.
