How do I lower the priority of a repository?

[Joerg Desch <news@jdesch.de>]

hi.

Including the official backport repository automatically uses a lower 
priority than the default 500. How do i have to do this? I've looked into 
the documentation of the "Package" file, but I can't find a priority 
setting?

Even the configuration inside /etc/apt doesn't include a special setting 
for jessie-backports.

My goal is to lower the priority of a private repository with backports 
lower than the one of jessie-backports (may be 80). I want to use these 
packages before the sponsored packages are uploaded to jessie-backports.

Thanks.