How to prevent indent on second line with packed itemize?

Question

How to prevent indent on second line with packed itemize instead of itemize? In other words I dont want "Major" to be indented.

\documentclass[letterpaper]{article}
\usepackage[empty]{fullpage}% http://ctan.org/pkg/fullpage
\newenvironment{packed_itemize}
{\begin{itemize}
\setlength{\itemindent}{-10pt}
\setlength{\itemsep}{-0.3pt}}
{\end{itemize}\vspace{-0.5em}}

\newenvironment{packed_inner_itemize}
{\begin{packed_itemize}}
{\end{packed_itemize}\vspace{0.25em}}
\newcommand{\ressubheading}[4]{%
\item\textbf{#1} \hfill #2\null\\
#3 \hfill #4%
}
\begin{document}
\begin{packed_itemize} 
\ressubheading{Name of Grad School}{City}{Major}{2010 -- 2012}  
\ressubheading{Name of Grad School}{City}{Major}{2010 -- 2012}  
\ressubheading{Name of Grad School}{City}{Major}{2010 -- 2012}  
\ressubheading{Name of Grad School}{City}{Major}{2010 -- 2012}  
\end{packed_itemize}
\end{document}

Answer 1

You should adjust the left margin, via \leftmargini instead:

\documentclass[letterpaper]{article}
\usepackage[empty]{fullpage}% http://ctan.org/pkg/fullpage
\newenvironment{packed-itemize}
{\leftmargini=15pt\begin{itemize}
\setlength{\itemsep}{-0.3pt}}
{\end{itemize}\vspace{-0.5em}}

\newenvironment{packed_inner-itemize}
{\begin{packed-itemize}}
{\end{packed-itemize}\vspace{0.25em}}
\newcommand{\ressubheading}[4]{%
\item\textbf{#1} \hfill #2\null\\\relax
#3 \hfill #4%
}
\begin{document}

\noindent Standard itemize:
\begin{itemize} 
\item{Name of Grad School}\hfill City\\Major \hfill 2010 -- 2012
\item{Name of Grad School}\hfill City\\Major \hfill 2010 -- 2012
\end{itemize}

\noindent New commands:
\begin{packed-itemize} 
\ressubheading{Name of Grad School}{City}{Major}{2010 -- 2012}  
\ressubheading{Name of Grad School}{City}{Major}{2010 -- 2012}  
\end{packed-itemize}

\noindent
Some text.
\end{document}

I have changed the name of your environment to packed-itemize to avoid the _ symbol, which to my eyes always denotes some mathematics. Also changed is the addition of a \relax after \\, to prevent nasty surprises if #3 begins with a square bracket [.

In general it is easier to define modified list environments via the enumitem package. Your example would then use:

\usepackage{enumitem}

\newlist{packed-itemize}{itemize}{1}
\setlist[packed-itemize]{leftmargin=15pt,label=\textbullet}
\newcommand{\ressubheading}[4]{\item \textbf{#1}\hfill #2\relax\\\relax #3
\hfill #4}