Edit: It looks like I accidentally dredged up an OLD post! Hopefully it helps anyone interested!
I have used the "back of the hobby knife" method as well, but since I picked up Bare Metal's Scribe Tool, I haven't looked back. The number one piece of advice I can give for cutting out doors, trunks, etc. (and for building models, in general) is to take your time. Doors with curves require you to go slow, otherwise, you won't be happy with the end result. The door jamb will need to be sanded/filled to ensure a proper fit.
After the door is out, you'll probably want to cut out the door pad from the interior (Otherwise, your door will open to the outside of your interior tub). Take your time. At this point, I paint and finish the door and door pad separately, then build the edge of the door and the hinge. When you attach the door pad to the door and finish it around the edge, it may not fit exactly where it's supposed to. Test fit everything and mock things up with tape or Elmer's before you commit to CA or cement. You have to make sure that the hinge fits with the rest of the interior, the door opens and closes properly, and the door pad fits back into the interior when the door is closed.
Long story short, everything that I cut off of a car adds 2-4 hours onto my build time(Your times may vary). Opening two doors and a trunk can easily double the time I spend on a model. I don't say this to discourage you, but to give you an idea of what it can take to make things right. You have plenty of resources in these forums if you run into trouble!
Some pictures from my '66 Impala:
Hinge view. This was my first, so I didn't paint before I glued. I ended up having to do a lot of masking. Now, I paint first.
Pic of the edge filler. I used sheet styrene and careful cutting/shaping. This gives the door a finished look and the support to hold the door pad in the correct place.
Finished trunk.
Finished door.
Links for hinge building that helped me: http://www.youtube.com/watch?v=GEiImNaNgFY,
Found the tubing at my LHS, and I used Household Amazing Goop (Kmart or similar) to attach the metal to the plastic.