Use local files during apt-get instead of internet?

I have installed 3.3 to HD using debian way. I have a Lucent winmodem that didn't work unti I downloaded and installed the source (what a lot of work!) Now I can do apt-get and everything OK. I'm still really new to this debian stuff- If I download big files from a friends faster net connection to my handy-dandy keydrive and put them in a directory, say /debian, how do I get apt-get to look there first and resolve any dependancies over my dialup when I'm connected? I want to install kernal sources, and other big stuff, but it's kind of a downer to see how long it will take. I've tried to update my sources.list file, but I can't seem to get the syntax right, because even with the .deb files there, it still looks to the net. Do I have to do an apt-get update to re-read the sources.list file when I make a change?

Check the man pages for apt-get (man apt-get), for how to specify the install source file.
Another way: browsing in Konqueror, right click on a .deb file should be an option to install it using kpackage. Kpackage is just a front end for apt-get.