I think the best way to welcome repeat visitors and encourage them into conservation is to have staff/volunteers on hand to engage with them. One of the biggest risks is that most staff/vols are doing a certain job such as feeding talks, feeding, cleaning, observing (in interaction enclosures). This means that whilst most are willing to talk, they often don't have the time to dedicate to it long term. I think the more personal interaction and conversation can reinforce a persons interest far more so - heck look at Zoo chatters who engage in such discussions and go for that little bit more than just what the viewing panel has written on it about a species.
The next level up from that is a strong volunteer program to try and get more people into helping out. Of course such a system should never be done at the expense of staff; but getting people more involved and showing them what goes in can be huge building blocks. Especially as things like zoo work or conversation are not always the kind of job that gets "advertised" to people (although its a lot better now than it was 20 odd years ago). Showing people how they can get involved, what happens, showing them where it can lead to work (even if its not work in zoos), how they can take part etc...
Even things like just providing a higher level of information through the zoo itself of through its shop - again its targeting the fewer rather than the masses, but its also filling a need in providing more depth to the experience and the message that the zoo is trying to convey. I oft feel that there's a huge amount of effort put into the initial pitch of conversation and zoo work, but that sometimes the follow up to that is far more muted.