custom usb install image

[nicholas <narusso@uchicago.edu>]

I've had success installing Etch from a USB stick prepared using
the easy way described in the manual[1], but I've had no luck
with the flexible way.
  [1] http://www.debian.org/releases/stable/amd64/ch04s03.html.en

Either the BIOS moves right past the USB stick, not thinking it's
bootable, or I get a single flashing cursor in the upper-left of
the screen. I think the difference between those two might have
to do with whether I tried install-mbr on the USB stick or not.

I want to use the flexible way so I can add a custom deb repository
that's bigger than the default 256MB image. I would be happy with
either a) creating a second partition to hold the repository, or b)
expanding the existing vfat to make more room, but I think the
default boot.img doesn't use any partitions at all, and I'm not sure
if expanding a vfat is even possible. The third obvious choice is to
build a boot.img from scratch using my own preferred size, but when
use the instructions from the manual, I never get a bootable image.

I'm concerned that I might be doing this wrong:
  copy the following files from the Debian archives to the stick:
   vmlinuz (kernel binary)
   initrd.gz (initial ramdisk image)

I've been using the versions here:
  http://http.us.debian.org/debian/dists/etch/main/installer-amd64/current/images/hd-media
Are those correct? The instructions are kind of vague.

Thanks,
Nick