I started with a test picture in Aperture priority mode (my default mode) to determine the correct exposure for the sky.
In this case, it was about 1/40 at f/2.8 (ISO 200). From there I moved to manual mode and locked in the settings. I then turned on my flash (SB-800) and set it to remote mode. I set my on-camera flash to Commander mode. I used TTL mode for the remote flash, with the on camera flash simply triggering the SB-800. Using the same exposure settings, I fired a test shot.
The sky retains the same color and light level, but the tree in the foreground is now lit as well. I thought it was a bit too much, so I set the TTL level for the remote flash to -1 in the camera's menu. All the other settings remained the same.
I like this shot a little better because it provides more contrast in the foreground tree.
This is a very easy setup to pull off. Simply set your exposure for the background, setup your flash for the foreground, and control the relative exposure of each using the Aperture/Shutter for background and TTL Over/Under exposure for the foreground.
I used a Nikon D300 and SB-800 here. The D300 functions in Remote Commander mode for the flash with TTL, so it is very easy. You can achieve the same effect with slightly more effort with a D40 or other cameras that don't have Remote Commander mode.
Using the D40, I set the on camera flash to manual mode at 1/128 power (or whatever the lowest is). You can then set the SB-800 to Remote using SU-4 mode. You have to control the flash exposure manually on the flash, but with a little trial and error you can pull it off.
