Suppose that the editor of your favourite journal has specified that section headings must be centred, in small capitals, and subsection headings ragged right in italic, but that you don't want to get involved in the sort of programming described in The LaTeX Companion (see TeX-related books; the programming itself is discussed elsewhere in this FAQ). The following hack will probably satisfy your editor. Define yourself new commands
\newcommand{\ssection}[1]{%
\section[#1]{\centering\sc #1}}
\newcommand{\ssubsection}[1]{%
\subsection[#1]{\raggedright\it #1}}
and then use \ssection and \ssubsection in place of
\section and \subsection. This isn't perfect: section numbers
remain in bold, and starred forms need a separate redefinition. Also,
this will not work if you are using the prototype NFSS with
LaTeX 2.09, because the font-changing commands behave differently
there.
The package sectsty provides an easy-to-use set of tools to do this job, while the package titlesec permits more advanced usage as well. (Titlesec comes with a second package, titletoc, which is used to adjust the format of table of contents entries.)
The fncychap package provides a nice collection of customised chapter heading designs. The anonchap package provides a simple means of typesetting chapter headings "like section headings" (i.e., without the "Chapter" part of the heading); the tocbibind package provides the same commands, in pursuit of another end.