In my references, I want to allow DOI's to break on a slash '/' character (or as a last resort on any character) to avoid underfull lines like the third reference below. Note that URL's are already breaking on slashes, and DOI's are already breaking on dashes.

\documentclass{article}
\usepackage[left=1.5in,right=1.5in]{geometry}
\usepackage{natbib}
\usepackage[colorlinks,breaklinks,urlcolor=blue]{hyperref}
\usepackage{doi}
\urlstyle{rm}
\bibliographystyle{sp}
\begin{document}
\nocite{Endriss:09} \nocite{Evans:80} \nocite{Szabolcsi:10}
\bibliography{references}
\end{document}
Here are some .bib entries:
@article{Evans:80, Author={Gareth Evans}, Journal={Linguistic Inquiry}, Number={2},
Pages={337--362}, Title={Pronouns}, Url={http://www.jstor.org/stable/4178164},
Volume={11}, Year={1980}}
@book{Szabolcsi:10, Author={Anna Szabolcsi}, Doi={10.1017/CBO9780511781681},
Publisher={Cambridge University Press}, Title={Quantification}, Year={2010}}
The bibliography style file can be downloaded as part of a zip file (containing also the actually used class file) here: Instructions for S&P authors using LATEX 2ε | von Fintel | Semantics and Pragmatics.
\penalty 0to the .bbl file manually produces a break, but messes up the link destination. – Noah Aug 23 '12 at 22:07