Tripod with long exposure - just expose for the sky.
* Mount your camera to the tripod.
* Switch it to M (manual) mode.
* Choose your aperture (f/8 - f/11 looks like it would probably be about right for this).
Here I'm going to just assume that adjustments on the XSi are made the same way that they are on the XT (they probably are, but if something doesn't seem right - check the manual).
In M, to change the aperture hold down the "Av +/-" button on the back of the camera and roll the wheel on top until you get to your desired aperture (f/8 - f/11)
* Point your camera at the sky.
* Now, looking at the meter at the bottom of the viewfinder (-2**1**V**1**+2), adjust the shutter speed (just roll the wheel on the top of the camera) until the moving needle at the bottom lines up with the stationary needle at the top.
It is now set to properly expose the sky.
(If you took 'too long' setting everything up, the camera might have gone into power save mode - just press the shutter button half-way)
* Compose your shot, focus, take the picture. (Using a remote can't hurt, if you have one.)
If it's getting dark (darker than 'dusk'), you may find that 30 seconds (longest exposure you can do without going into Bulb) is not long enough.
If that happens, there are a few things you can do.
You could increase the ISO, you could use a larger aperture (smaller f/#), or you could figure out how many seconds the exposure needs to be and expose that long in Bulb.
On my XSI i couldnt find any moving needle that would line up with a stationary needle.
T...The shot was not very dramatic, maybe it wasnt dark enough...