add a line for testing to your /etc/apt/sources.list.
e.g. if you have the following sources.list already
deb http://your.favourite.mirror/debian/ stable main non-free contrib
then yank and paste it and change the copy to
deb http://your.favourite.mirror/debian/ testing main non-free contrib
edit or create /etc/apt/preferences to assign Pinning priorities, like so
Pin: release a=testing
Pin: release a=stable
run "apt-get update"
install apache2 from testing (along with all required dependancies) with:
apt-get -t testing install apache2
from now on, when you run apt-get upgrade or apt-get install it will install packages from stable by default but you can override that at any time with "-t testing" to install specific packages (or even a full upgrade) from testing.
you can do the same thing with "unstable" too. just add it with a lower priority than testing.
personally, i would use unstable instead of testing. in my experience, testing is often more broken than unstable because there are often very long delays in getting crucial updates to migrate from unstable to testing - all it takes is one new (or duplicate) bug to be reported or a slightly new revision to be uploaded to unstable, and the automatic migration of a package from unstable to testing will be delayed for another week or two or more...which is more than enough time for yet another bug report or updated package to be uploaded to unstable to reset the delay all over again.