I do night photography reasonably often (as seen from the flickr link below) and i'll tell you what I do.
Granted, I use better equipment but that just makes things easier, it doesn't mean its impossible on lower end gear.
First and obvious things first:
Make sure the moon's not up yet, or it's already set, or it's a new moon. Otherwise it looks like daylight with a couple stars in the sky.
Tripod, At least one fully charged battery, Cable Release, bubble level if the camera doesn't have it built in, Shoot RAW, and an eyepiece shutter/cover are a must. When you're shooting at night, you've got make sure you've at least got your bases covered.
So once you're set up, cameras locked onto the tripod, cable release plugged in, with the fully charged battery and your eyepiece shutter closed, you're ready to go.
I use live view to focus, but with the 18-55mm that might be a little difficult since it's not very fast. So what you do is focus the lens to infinity, but then pull
ever so slightly back. Since only the sigma lens has focus scales on it, this might be tricky.
Once you've focused, compose the picture, again, i use live view, but you might have to use the viewfinder, or guess and check it. The level will help keep the camera straight. You should also set your WB to something pretty cool. I usually start off at about 2700Kish, but if you can't do direct Kelvin, just leave it on tungsten.
OK, so you're composed, focused, locked down, have the cable release on, set your WB about right, you're ready to shoot your first picture.
Set the ISO to the max. Not sure what the 450D does, but crank it. I do 25000 on my camera, you might be able to do 12000. It's going to look like Dog S***, but that's ok. We just want to make sure our composition and focus is good and we have a picture to gauge exposure from.
So for example, i'll post one of mine:
Lost Lake, ISO 25600, 4 seconds, f/2, 50mm:
OK, cool, so the exposure at ISO 25k is 4 seconds and f/2. Perfect!
Now we crunch the numbers down to the equivalent exposure at a lower ISO like 200 and stop the lens down a bit too, I did f/2.8 because that's more than enough DOF for this.
So the same exposure at ISO 200 and f/2.8 would be about 32ish minutes. That's where the cable release and the eyepiece shutter comes in. For such long exposures the camera won't automatically go past 30 seconds, and the eyepiece shutter keeps light from leaking in.
So the same thing as above, but ISO 200, 32ish minutes, f/2.8 looks about like this:
Dynamite!
So once you do this a couple times, within about 10 minutes of putting your tripod legs down, you'll be doing the actual exposure, and it will be dead on, everytime. It's actually pretty easy once you get the hang of it.
Also, make sure the moon isn't up, because otherwise, this is what happens:
Looks like sunrise, except the water's blurred, the exposures are much shorter, and there's only a few stars in the sky. HUGE difference.
Hope that helps!