Ever since I've been going to Eastercons, there have been panels along the lines of "How do we get more young fans coming to Eastercon?" or "Where are the conrunners of the future going to come from?" Broadly speaking, you can replace these questions with "How do you get new people to come to your con for the first time", "How do you get them to come back to your con again in future years," and "How do you get them to volunteer for activity X?"
The panel-parity discussions often involved comments (or implications) along the lines of "but the Eastercon demographic doesn't support that." Well, if your demographic is skewed in one direction, you need to reach out in the other direction, in an attempt to bring in new members to balance that.
Once you bring in a new member, you have to try to convince them that the event is worth a second go. If they have fun, they're more likely to come back. If they're treated like crap, this is less likely to be the case.
At the most basic level, don't actively attack them or the genre aspects about which they're passionate. You might not like movie A/author B/hobby C/medium D, but they do. Accept and engage, rather than attack and dismiss.
New members are unlikely to return if everyone ignores them, or pretends they don't exist. Some new members are out-going extroverts happy to break into a clique and see what happens. Some are at the other end of the scale, and turning up alone to an entirely new, immersive, overwhelming environment is already pushing the boundaries of what they feel up to doing; breaking into an apparently closed culture where everyone already knows everyone else is beyond them.
Dave Lally noted that some cons provide "it's my first con" badges, but I don't think that's so good an idea. It's placing the onus onto the newcomer, asking them to single themselves out as inexperienced and uninformed, an outsider. Analogy for the real-ale fans: imagine if it was suggested that you should wear just such an indicator for your very first time in a pub. Sound fun?
(There's also the problem of how such a newcomer finds out that they should be doing this, since they're uninitiated.)
Instead, I suggest that we should extend the extra effort on behalf of the newcomer, to make it easier for them. We should have badges (or other visible indicators) suggesting that, right now, we don't mind if random strangers come up to us and start talking (whether those strangers are first-timers, or have been coming for years, but still haven't made all that many friends). And we can always take the badge off when we need a break.
I can understand, btw, that for many members, Eastercon is the time when they catch up with friends they haven't seen since the last Eastercon (or since one prior to that). I can understand that you might not have time to hang out with everyone you've missed, and you'd like some time just renewing acquaintances. That's fine. But try to spare a little for the newcomers, too.
Volunteering's frequently put forward as the recommended way to make new friends at one's first con, and indeed it was raised at the Minorities panel. Well, sometimes it works, and sometimes it doesn't. I've read con reports by volunteers in Tech, who note that everyone in Tech was just too busy to talk and make friends. I've read con reports by people who've volunteered for gophering/stewarding, and they've sat on a chair for hours checking badges as people go in or out of a door, but they haven't talked to people. And I've been in gopher holes watching some people engaged in conversation, while the shy gopher sits for hours without saying a word.
Points were made in the Minorities panel that the commercial media cons cast their net more widely (most notably via ads in magazines like SFX). They have a financial incentive, of course, but they're reaching areas that promotional material for Eastercons does not. They also tend to have far less programme, putting everyone together into the same hall - so it's easier to mingle/discuss. Moreover, because media cons are more focused (autographs/photos of actors for particular TV shows), pretty much everyone there has a common interest. Compare that to Eastercons, where there is not only the wider spread of topics across the programme, but also the disparate sub-cons that comprise Eastercon (lit, media, filking, art show, costuming, gaming, real ale, etc.), all of which serve to partition the members both physically and psychologically.
And that brings me onto something else: Eastercon worships the longtimer.
Don't get me wrong: it is good to recognise and reward the hard work that many have put in over the years. The Doc Weir award is an excellent example of this.
But there's frequently an entire stream and/or entire room dedicated to glowing nostalgia about the good old days. There's the "fan guest of honour". There's the bizarre interpretation of "fandom" or "fan" or "fannish" as meaning "been around Eastercon a long time" or "did things the way we used to do them." The longer you've been around Eastercon fandom, the more you are explicitly valued by the Eastercon regulars. Implicitly, that tells the newcomer that they are less valued. By all means have such items, but be aware of the impression they give, and find ways to balance that impression. If you want new members to come back again, you should help those newcomers to understand that they too are valued - and not just in financial terms. Their enthusiasm is needed, as are their new perspectives, thoughts, opinions, skills and experiences.
Reach out. Welcome in. Listen. Respect.
Or, as emmzi and gaspode said in the Olympus opening ceremony, be excellent to each other.