Fwd: how to apply patch?

I have a development version & a production version of my website.
There are a few essential differences between them that I want to
always stay different (like path constants, etc). When I make changes
on the dev version and test it out and they work and am ready to copy
them to the production (www) version, how do i do that without
annihilating the essential differences that i want to keep? I was
thinking of creating a patch or branch or something with the
differences. Then, copy dev to pro, then reapply/rebase the essential
differences to pro that got wiped out. if i rebase though, i want to
be able to rebase again on the next edit.
what's a good way to do this?

My dir structure:

example.com
  www  # production site
  dev   # development site

Via DreamHost control panel:
www.example.com points to example.com/www/
dev.example.com points to example.com/dev/

Why don't I just have a branch called dev? Maybe I should... Advice?
(Anyway, it's nice to be able quickly, simultaneously open
corresponding files from both www & dev in textmate.
I guess I could do that with like git show master:www/index.html |
mate or something, but then it opens in a new textmate window. it's
just not the same. I guess there's always the possibility of editing
www/index.html when I actually want to be editing dev/index.html, but
then again, it's also possible to be editing the wrong branch.)

My .git/config file has the following aliases for deployment:

# deploy dev
ddev = !rsync -avi --copy-unsafe-links --exclude='*.DS_Store'
--delay-updates --delete-after --delete-excluded -e ssh
~/Sites/example.com/dev/ acani@acani.com:Sites/example.com/dev/

# deploy www
dpro = !rsync -avi --copy-unsafe-links --exclude='*.DS_Store'
--delay-updates --delete-after --delete-excluded -e ssh
~/Sites/example.com/www/ username@example.com:Sites/example.com/www/

Thanks!

Matt