`holding back' packages?

[Krzys Majewski <majewski@cs.ubc.ca>]

1. How can I `hold back' packages so Debian doesn't upgrade them? 
2. Does apt-get/dpkg always overwrite existing files, or only if their
timestamps are older than the corresponding file in the .deb? 

I run  a customized potato box.  Usually I keep  custom stuff separate
from Debian by installing it in  /usr/local. This was not the case for
XFree86 version  4. Thinking I had  some time to kill  this morning, I
foolishly did a `apt-get upgrade', which "upgraded" the X packages
to  whatever  Debian thinks  is  the latest  version  of  3.3.6. My  X
installation got  destroyed. I'd like  to prevent this  from happening
again ;) 

-chris