I agree completely that we need some guidance, even if it's not perfect, so that people can quickly decide whether their question is on-topic or off-topic. Iterating will be key here - if the guidance we provide doesn't work, we can easily switch to a different wording to try it out.
When the Pro Tempore Moderators are elected, they will be able to edit the Help Center (and Tour) to some extent. Hopefully, this will provide a little bit of direction as to what is on-topic here.
Of course, you might point out that very few people read the Help Center, and quite a few don't even bother to read the Tour, which (if I recall correctly) is presented to you on sign-up if you're new to Stack Exchange. Judging by the badge, only 12.5% of users actually read the tour, so it's doubtful that they had any guidance at all about what's on-topic.
It might be helpful to leave a comment if someone's asked an off-topic post, to this effect:
Welcome to the site! Your question doesn't seem to be on-topic here, because _________. Take a look at the [tour] for a quick introduction if you haven't already, and don't forget to edit your post if it's closed, so that it can be reopened.
(note that I used the [tour] magic link, which will automatically link to the site tour in the comments)
However, don't harass people just because they haven't read the tour. I occasionally see comments on Stack Overflow saying things like:
Why should we help you if you're too lazy to even read the [tour]?
That's clearly not what we want here - flag things like this if you see them, and comment more constructively (in other words, Be Nice!)
I think it's inevitable that some people will misinterpret the scope, even with all the resources we provide. In these cases, all we can do is close, comment and encourage the user to find out what we accept here.
I am a little concerned though that this is a rather fundamental problem with the site scope, and we're having to close a lot of questions as off-topic. Over the whole beta so far, we've closed 8% of questions as off-topic. In the last two weeks (essentially the entire public beta), we've closed 11% of questions as off-topic. It will be worth monitoring this to see if it gets any worse - if so, further action might be needed.
We could ask if it's possible to get an advice screen before asking a question, like Stack Overflow has. I don't think this is implemented at the moment (but I may be wrong!)
An alternative idea could be to create a custom 'new user guide' like Mechanics.SE did. Although a lot of people don't read the guidance, if we are keen on providing more information, this could be the route to take.
So, in brief: