Henrik,
The trick is exactly the same as during the day. Look down the runway at the overall shape of the runway lights - don't look too close to the plane, and don't look for detail.
The problem is that many pilots don't look down the runway. During the day, there are enough visual clues about what's going on that if you don't look far enough ahead, you'll be able to pull off an ok-ish landing, although you won't ever really have a feel for what's going on. But at night, all those clues disappear, and the only way you'll be able to judge what's happening in the flare is to look well ahead.
Once you've mastered this technique at night, use it during the day too, and you'll find your landings become much more consistant. At least, that's the way it worked for me!
Have fun,
FFF
-------------