Persistent URLs (also called PURLs, stable URLs, persistent links, or durable links) are internet addresses that remain constant.
Many databases vendors (e.g., Ebscohost, ProQuest, Gale, JSTOR) include a stable or persistent URL in the record. When you find an item, look for the stable URL. It's often found under 'cite' or 'permanent URL' links. Only use the URL in the address bar as a last resort!
Some databases are extra nice and include the proxy prefix (see above) but if they don't, be sure to add that to the beginning of the stable URL.
Here's how a permanent URL looks in JSTOR:
So a stable URL for this record that would work both off-campus and on-campus would be:
http://ezproxy.stolaf.edu/login?url=http://www.jstor.org/stable/40645554