Suppose you want your top-level enumerates to be labelled `I/', `II/',
..., then give these commands:
\renewcommand{\theenumi}{\Roman{enumi}}
\renewcommand{\labelenumi}{\theenumi/}
The possible styles of numbering are given in Section 6.3 of Lamport's
book (see TeX-related books). Both
\theenumi and \labelenumi must be changed, since
\theenumi is used in cross-references to the list.
For lower level enumerates, replace enumi by enumii, enumiii
or enumiv, according to the level. If your label is much larger
than the default, you should also change \leftmargini,
\leftmarginii, etc.
If you're running LaTeX2e, the package enumerate.sty (part of
macros/latex/packages/tools) offers similar facilities. With
enumerate.sty, the example above would
be achieved simply by starting the enumeration
\begin{enumerate}[I/].