Tends to go in swings and roundabouts. We get peaks and troughs due to seasonal reasons, a new game comes out, the students are here or away, people get bored or jaded, etc.
One thing that drove some interest for us was having a core-set-only tournament. That brought some people out of the woodwork and for some n00bs involved.
There's no accounting for people though. This week at games night there were four of us, a couple of weeks ago it was 16. There's a GNK tournament at the weekend, but who knows how many we'll get!
Having events and special features is a way to get people out of their homes. Folk often find an excuse to avoid regular nights as "I can always go next week" becomes a feature.