Correct way to use an older version of a package in Ubuntu?

[Adam Funk]

What's the correct way in Ubuntu to downgrade to a previous version of
a package and keep it that way for a while?  

Last time I tried to do this (possibly using one of my old Debian
command-line habits), I kept getting annoying messages in the update
manager telling me to do a "partial upgrade" which did nothing.

Thanks,
Adam