How would I write a macro (to be placed immediately before every URL in a bibliography) whose effect is to tell TeX

1. This is a good place to insert a line break, but breaking the line here is not required.
2. If (and only if) you break the line here, insert a \hfill (or maybe a \par, I'm not sure which makes more sense) immediately before the line break.

I tried \discretionary{\hfill}{}{}\penalty-500\relax which gave me a bunch of "Improper discretionary list" errors, and I also tried \discretionary{\par}{}{}\penalty-500\relax which didn't give me any errors but also didn't seem to have any effect.

(Also I'm not sure if \penalty is how you encourage a line break.)

(What I really want is for TeX to insert a paragraph break before the URL if and only if the URL doesn't fit onto the space remaining in the current line, but -- after that break happens -- not interfere with url.sty's intra-URL line breaking. But I suspect this is not even possible, whereas what I described above, I'm pretty sure is possible, I just don't know how to do it.)

EDIT: Minimal-ish example of overfull hboxes with TH's currently suggested code:

\documentclass[twocolumn]{article} \usepackage[hyphens]{url} \usepackage[letterpaper,margin=1in]{geometry} \begin{document} \urlstyle{same} \begin{thebibliography}{10} \let\oldurl\url \makeatletter \renewcommand*\url{% \begingroup \let\do\@makeother \dospecials \catcode`{1 \catcode`}2 \catcode`\ 10 \url@aux } \newcommand*\url@aux[1]{% \setbox0\hbox{\oldurl{#1}}% \ifdim\wd0>\linewidth \strut \ \vbox{% \parindent=0pt \kern-\lineskip \raggedright \strut\oldurl{#1}% }% \else \hskip0pt plus\linewidth \penalty0 \box0 \fi \endgroup } \makeatother \bibitem{thinkermade08focus} ThinkerMade. (2008) {How to Tell if a User is Signed in to Facebook and Other Services}. Blog entry. \url{http://replay.waybackmachine.org/20081020072934/http://www.thinkermade.com/blog/2008/07/how-to-tell-if-a-user-is-signed-in-to-facebook-and-other-services/} \end{thebibliography} \end{document} 

Complaint:

(./argh.tex (/usr/share/texmf-texlive/tex/latex/base/article.cls Document Class: article 2007/10/19 v1.4h Standard LaTeX document class (/usr/share/texmf-texlive/tex/latex/base/size10.clo)) (/usr/share/texmf-texlive/tex/latex/ltxmisc/url.sty) (/usr/share/texmf-texlive/tex/latex/geometry/geometry.sty (/usr/share/texmf-texlive/tex/latex/graphics/keyval.sty) (/usr/share/texmf-texlive/tex/generic/oberdiek/ifpdf.sty) (/usr/share/texmf-texlive/tex/generic/oberdiek/ifvtex.sty)) (./argh.aux) *geometry auto-detecting driver* *geometry detected driver: pdftex* Overfull \hbox (20.5556pt too wide) in paragraph at lines 40--43 [] [1{/var/lib/texmf/fonts/map/pdftex/updmap/pdftex.map}] (./argh.aux) ) 

This is not just a cosmetic problem; the URL extends into the right margin.