Pin: for a local repository

["Nikita V. Youshchenko" <yoush@cs.msu.su>]

Hello.

I've created a local repository of some locally rebuilt packages.
I've added

deb file:/home/nikita/exp/packages sid/

to /etc/apt/sources.list, and places .deb files and Packages file (generated 
by dpkg-scanpackages) there. It works: apt sees the packages, can install 
them, etc.

Now I want to configure /etc/apt/preferences to fine-tune, which packages 
from where apt should take.
But I can't figure out, what to write to Pin: line of the preferences file 
and/or to Release file. I fail to understand man apt_preferences :-(.

Please help (or point me to appropriate docs). Thanks.