I have recently finished shooting a live concert using the D800 and found that the best audio quality was achieved by using the built in microphone of a Zoom H4n without the need to sync in post. First I set the recording level on the Zoom H4n during the sound check at around -12db and the headphone level at 70. Using a Sescom H4n cable I connected the zoom from the headphone out to the mic input of the camera and set the D800 recording level quite low around 5 which gave what i thought a good recording without any hiss, distortion etc. I listened to the audio that the camera had recorded and didn't feel I needed to use the audio recorded by the zoom although this was more dynamic. There are other, cheaper recorders available. Its all about being able to raise the recording level of the external mic while keeping the cameras recording level as low as possible thus keeping the hiss to a minimum.
Here is a couple of tracks I recorded using the above setup.