I use a 350D for some shots and do it this way (since it doesn't have live view either).
Pick something that is easy to focus on. A street light or a neighbors house works well. It doesn't have to be too far away - more than 20ft for your lens at 18mm or 100ft at 55mm. Focus as best you can through the viewfinder. This will get you very close.
Now point at the stars you want to photograph and do a test shot with the highest ISO you have. Review it on the LCD zoomed in as far as you can, moving around to a bright star. If it is slightly out of focus, move the lens focus a tiny amount in one direction, remembering which direction you moved it. Do another test shot and zoom in on the display again. When you press the button to go back to the previous shot, it remains zoomed in so you can compare the two shots quite easily. If it got better, move the lens in the same direction. If it got worse, move it in the other direction.
You can zoom in much further on review than you can in live view and it only takes a minute or two to get good focus.
Back the ISO off, adjust the shutter speed and go for it.