I might be in the minority here, but I feel that if people had conversations in relevant threads rather than in the shoutbox, we'd have better engagement on certain topics - with or without new membership. Call me Mr. Old-Fashioned.
Also, while I'm not against encouraging new blood to sign up, I do like the small, exclusive feel we have now. Drumming up new members in the run-up to new PTA seems to make sense, but could also consider FB ads targeting people who follow stuff like Mubi, Film Comment, etc. to ensure we have pedigree snobs ;)
Do we have any analytics to see what sort of searches most commonly bring newbies here? JB's Inland Empire essay seems to frequently encourage new people to visit and engage, for example. If we had some other good thought pieces like that we could use as the promotional item - it's far less spammy to see a FB ad which is sharing an article like that than simply a promo to join a forum. GT has written some good stuff too, and I'm sure there's others.
He held on. The dolphin and all the rest of its pod turned and swam out to sea, and still he held on. This is it, he thought. Then he remembered that they were air-breathers too. It was going to be all right.