How do I upgrade to testing from sarge?

Edit the /etc/apt/sources.list, change every "stable" or "sarge" to "testing" or "etch". I prefer to use the names (i.e. sarge, etch, etc) because then when the next release happens, you won't all of the sudden be in testing until you want to be.

Then run apt-get update and apt-get dist-upgrade.

You may find it a safer upgrade to upgrade the kernel 1st from etch site. Etch kernel are called linux-imageXXX instead of kernel-image. Etch changes to using udev/hal which require 2.6.12+ kernel which is default with etch. When I tried apt-get update apt-get ugrade w/o doing kernel 1st I got error messages galore.Expect message after upgrade that tells you to purge hotplug. With new kernel you will need to redo nvidia install if you use nvidia card. XF86 changes to Xorg in etch.
Check sources lists at top of this forum for etch sites that should include testing security site as well. If you use marillat site then you should change that to etch as well. And expect gpg error messages from etch sites. They dont affect upgrades but several posts in this forum tell how to fix those.