Stay logged in on a Plone site
In Plone 2.5x, to allow users to remain logged in on a Plone site, all you had to do was go to your site ZMI > portal_properties > site_properties > auth_cookie_length > set it to a number of days before it expires.
But Plone 3 came with new authentication, and this stopped working. Eventually a robust fix was added, and you can now set this up as follows:
ZMI > site acl_users > session > properties > Cookie lifetime (in days) > set to whatever you want. Easy when you know how.
Here are some useful links which explain what happened.
Hands up if you love Plone's brilliant way of documenting stuff.