Lots of good advice and things try here, but whilst faster lenses are generally good for night shots, you will get a limited improvement in AF performance so you will hit the point where it's too dark to AF.

Faster lenses will also give you a brighter viewfinder which enables better manual focus, but again up to a point, with the default focus screen you see no improvement past f/2.6

Lighting the subject to focus is a good idea but not always possible, using the depth scale isn't possible in your case so I'm going to suggest something that's not been mentioned so far.

Focus bracketing

Just as exposure bracketing uses multiple exposures to overcome metering problems, focus bracketing uses multiple shots to overcome AF problems. Start with your best guess for the focus and shoot two shots either side by moving the focus ring slightly and then examine the shots on the LCD. It's time consuming to work this way for sure, but it's a good technique to employ as a last resort. I only wish camera bodies included this as a feature as it would become faster and more accurate.

If this still doesn't work for you consider recomposing to increase your depth of field to give yourself a better chance!