It's not as practical as it was in the olden days, since packages have many more dependencies these days, but I used to use http://packages.debian.org/ to figure out which packages had to be downloaded to satisfy dependencies. ldd is also handy if you want to keep things to an absolute minimum.

As for the apt tricks, an ubuntu site suggested:

apt-get download package:architecture

If you're trying to get debian packages from ubuntu, I'm pretty sure that you only need to change the sources.list file. Caution is definitely advised though, since I imagine that you could really mess up your system that way.