Superscripts before a letter in math

For a quick and dirty solution, try putting an empty group {} before the ^ symbol.

\documentclass{article}
\begin{document}
\[
^gp = {}^gR^l_l+{}^go_l
\]
\end{document}

For a better method, check out the mathtools package, which provides the \prescript command.

In addition to the other solutions already presented, here is another one:

The (super-small) leftidx package provides left and right super- and subscripts to be typeset by means of \leftidx that is defined as follows:

\newcommand\leftidx[3]{%
  {\vphantom{#2}}#1#2#3%
}

It therefore provides

\leftidx{<left indexes>}{<object>}{<right indexes>}

where <left indexes> and <right indexes> are similar in context to regular super and subscript usage. Here's an example:

\documentclass{article}
\usepackage{leftidx}% http://ctan.org/pkg/leftidx
\begin{document}
\[\leftidx{^g}{p}{} = \leftidx{^g}{R}{^l_l}+\leftidx{^g}{o}{_l}\]
\end{document}

The first solution suggested by @Ian Thompson may have some alignment issues (depending on what you actually have as super-/subscripts). But there again, a work-around is:

\begin{equation*}
{}^{14}_{6}\text{C}
\qquad \text{versus} \qquad
{}^{14}_{\phantom{1}6}\text{C}
\end{equation*}

NOTE: Example taken from The Not So Short Introduction to LaTeX2e (page 70).