Here's what I found (thanks to those who replied):
It seems the /etc/apt/preferences file is a feature not yet in the stable
release of apt-get (version 0.3.19). I found some information about the
preferences file in a how-to document based on version 0.5.3 (chapter 3 of
http://www.debian.org/doc/manuals/apt-howto/), although I was still a little
unclear about how to selectively upgrade a package using this feature. My
guess is to use the example in the document for how to downgrade all
packages to the stable release versions, then insert a specific entry for
the latest version of, in my case, cproto.
Anyway, this is what I did that worked:
1. Added following line to sources.list:
deb http://http.us.debian.org/debian unstable main contrib non-free
2. apt-get -s update
The -s (=--dry_run) above seemed to have no effect because the cproto
package *was* found in the next step (without update it is not found).
3. apt-get -s install cproto
This showed me that apt-get would not upgrade or install any new packages
except for cproto.
4. apt-get install cproto
This installed the new package.
5. Commented out the line added in #1
I did this so that the next 'apt-get upgrade' won't upgrade me to the
unstable release of all the packages I have installed.
-Kris
