The general issues of adjusting the appearance of section headings are pretty complex, and are covered in the question on the style of section headings.
However, people regularly want merely to change the way the section number appears in the heading, and some such people don’t mind writing out a few macros. This answer is for them.
The section number is typeset using the
LaTeX internal
\@seccntformat command, which is given the “name” (section,
subsection, …) of the heading, as argument. Ordinarily,
\@seccntformat
merely outputs the section number, and then a \quad of space:
\renewcommand*{\@seccntformat}[1]{%
\csname the#1\endcsname\quad
}
Suppose you want to put a stop after every section (subsection,
subsubsection, …) number, a trivial change may be implemented by
simple modification of the command:
\renewcommand*{\@seccntformat}[1]{%
\csname the#1\endcsname.\quad
}
However, many people want to modify section numbers, but not
subsection numbers, or any of the others. To do this, one must make
\@seccntformat switch according to its argument. The following
technique for doing the job is slightly wasteful, but is efficient
enough for a relatively rare operation:
\renewcommand*{\@seccntformat}[1]{%
\csname the#1\endcsname
\csname adddot@#1\endcsname\quad
}
which uses a second-level command to provide the dot, if it has been
defined; otherwise it merely appends \relax (which does nothing
in this context). The definition of the second-level command (the
version for the section, here) specifies what to put after a
section number, but it could be used to put anything after it:
\newcommand*{\adddot@section}{.}
Note that all the command definitions above are dealing in
LaTeX internal commands, so the above
code should be in a package file, for preference.
The Koma-script classes have different commands for specifying
changes to section number presentation: \partformat,
\chapterformat and \othersectionlevelsformat, but otherwise
their facilities are similar to those of “raw” LaTeX.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=seccntfmt