Geoff has give you some good advice; the other trick with shallow depth of field shots is to ensure that the eye closest to the camera is in sharp focus, if other parts of the image are a bit soft, it will be okay. If the eye is not sharp, then the image will not "look right".
As you are shooting with a 18-55mm lens; shoot at the 55mm setting. That will do two things for you:
a. The longer focal length will result in less distortion in the face; and
b. The longer focal length will give you a narrower depth of field and throw the background out of focus more easily than a wider angle.
I assume this is along the lines of what you are looking for?
Karo 01 by
The Grumpy Diver, on Flickr