The comment package provides this functionality via the environment comment. One could also specify a new comment environment that could be included/excluded. See the comment package documentation for more information.
A more straight-forward way is to use conditionals in LaTeX:
- Define a
\newif;
- Set the value to
true to show the comments (say);
- Set the value to
false to hide the comments (say).
Here's a minimal example that defines \ifkeepremark as a \newif and sets it to true (\keepremarktrue) or false (\keepremarkfalse) depending on whether you need it or not:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\newif\ifkeepremark
\begin{document}
\section{Inner Loop (for $l$)}
We begin with inner loop - s.t. the given constraints, we find the optimal
level for individual labour supply in equilibirum,
\begin{equation}
l=l^{\star}.
\end{equation}
\keepremarktrue
\ifkeepremark
I want to keep this remark.
\fi
\keepremarkfalse
\ifkeepremark
When necessary, I would like to typeset this remark on or off.
\fi
\end{document}
It is possible to maintain the remark environment look using the environ package. It allows for environment definition in a control sequence style:
\usepackage{environ}% http://ctan.org/pkg/environ
\newif\ifkeepremark \keepremarktrue
\NewEnviron{remark}{\ifkeepremark\BODY\fi}
You could then still use
\begin{remark}
Here is a remark.
\end{remark}
together with the "switches" \keepremarktrue and \keepremarkfalse to selectively keep/discard remarks. The default value is \keepremarkfalse, which I've overridden with an issue of \keepremarktrue. That is, by default, all remarks will be visible.
You could also define your own remark environments based on other/alternative \newifs that you can turn on/off in the preamble (for a global modification).
commentpackage. Is it possible to have the standard look of Remark [or anyenvironment] whilst using the requested feature? – Ash Feb 25 '12 at 00:51environpackage. That allows you to define aremarkenvironment that you can use the conditionals with. – Werner Feb 25 '12 at 00:59