I'veused the 11-18 quite a bit for interiors. My method:

1. If possible use a tripod. It allows you carefully adjust the framing and if necessary use a slow shutter speed
2. Make sure the camera is not pointing up or down but level so the walls are parallel with the edges of the frame. The photo should not appear to be done with a wideangle
3. Don't zoom out too far so you have acres of empty wall with a bit of interesting detail huddled in the middle
4. For pictures with no light coming in from windows, use ambient lighting and a slow shutter speed (see point 1)
5. For pictures with light coming in from windows you need to supply fill-in lighting so that the windows are 1-2 stops overexposed compared with the room. I usually use a cobra flash bounced off the wall above and behind the camera. Be sure not to include any of the area lit directly by the flash in the image. I use a fairly powerful cobra flash (examples for Nikon = SB800, SB900, SB600). Do a test to check the effect