Here is an example Latex file where I'm experimenting with various newcommand and renewcommand capabilities. The file actually contains six examples, and in each step I add one more Latex feature that is a little harder than the previous step. In the last several examples I start passing parameters into my commands. After that, I start using the "html" package (html.sty) and the "ifthen" package (ifthen.sty) to do some decision-making in my last several newcommand/renewcommand examples.

Without any further ado, here is the sample Latex code:

\documentclass[a4paper,11pt]{article}
\author{Al Alexander}
\title{}

\usepackage{html}
\usepackage{ifthen}

\begin{document}

%-----------------------------------------
% (1) a simple test to get started.
%     replace the "enumerate" environments
%     begin/end tags with our own
%     commands.
%-----------------------------------------

\newcommand{\be}{\begin{enumerate}}
\newcommand{\ee}{\end{enumerate}}

% now use the new command
\be
  \item Item 1
  \item Item 2
\ee

%-----------------------------------------
% (2) Create a new command named "\atest"
%     and just use it to replace some text.
%     Start fooling around to make the 
%     syntax more readable.
%-----------------------------------------

\newcommand{\atest}
{
 and seven years ago
}

Lincoln said "Four score \atest"

%---------------------------------------
% (3) renew/re-use the same command name
%---------------------------------------
\renewcommand{\atest}
{
 our fathers set forth
}

\atest

%---------------------------------------
% (4) pass in an argument now. start
%     getting ready to do what I really
%     want to do, which is to accept an
%     argument and make a decision about
%     what to print. note that the "<"
%     and ">" symbols don't really print
%     right with pdflatex, but that's not
%     what I'm after in the long run, so
%     it's no big deal.
%---------------------------------------
\renewcommand{\atest}[1]
{
IMAGE: <img src="{#1}" border="0">
}

\atest{images/test.jpg}

%---------------------------------------
% (5) Same thing, but use the html pkg
%     to make a decision.
%---------------------------------------
\renewcommand{\atest}[1]
{
\latex{HTML PKG: Do the Latex thing to include this image: {#1}}
\html{HTML PKG: <img src="{#1}" border="0">}
}

\atest{images/test.jpg}

%---------------------------------------
% (6) Similar, but use the ithen pkg
%     to make a decision.
%---------------------------------------
\renewcommand{\atest}[2]
{
  \ifthenelse{\equal{#1}{html}}{IFTHEN: <img src="{#2}" border="0">}{}
  \ifthenelse{\equal{#1}{pdf}}{IFTHEN: Do the Latex thing: {#2}}{}
}

\atest{html}{images/test.jpg}

\end{document}