The approaches to teaching language well have undergone major changes and heated debates in the field of second language acquisitions

How to select authentic materials
The first criterion to me is that they are motivating or that the exercises that can be done with them are motivating. Road signs such as 'End of urban clearway' may say nothing to non-drivers, and even to drivers may yield little that can be done in the classroom. The same with petrol pump signs or the notices in railway trains. But something like the train timetable or the ticket presents things that are relevant to the students' knowledge of the foreign culture or to their functional needs when visiting it; they may also be used for various types of simulation activities and information processing activities in the classroom. Newspaper headlines, and the articles beneath them, also may give more general interest; graffiti may give an insight into a more popular side of life.
The second linked criterion is that they are not too ephemeral. If they are already of historical interest, there seems little point in using them. Either the teacher has to use things which are as up-to-date as possible or which have a timeless quality about them. It is still possible to discuss the Minimum Lending Rate (MLR), while it is no longer possible to discuss the Rampton brutality except as a thing of the past.
Thirdly, they have to be organised in some way. There is nothing worse than entirely disconnected bits of authentic language that are not linked to other aspects of the teaching. The obvious way to make this link is through themes; most of the examples I've quoted could be linked by the theme 'Travelling' because that was what I happened to be doing on October 8th. But they can be organised around many other themes, whether functional, such as 'shopping', 'banking', 'getting a job', 'eating out', or general discussion, 'is transport degenerating?', should smoking be banned in public places?', or in some other way. The authentic materials are not the point of the course, but a way of achieving that point. Fourthly, they have to be selected in terms of their language and content. This may seem like a contradiction: anything a native speaker says is by definition authentic, so how can we possibly censor it? But there are many things a native speaker says that I do not want in my classroom. Sometimes this is a question of language; letters to the local newspaper in England are often written by people who are unaccustomed to writing but are highly moved by some local issue; their language tends to be rather strange, often veering towards unnecessary pompousness, hypercorrectness, or even ungrammaticality. Sometimes, however, it may be the actual content of what is said that is objectionable. I deliberately included in my examples the graffiti 'Be a woman and wear a skirt', but would you use it in your classroom? Some people are sexist, racist, or have other types of prejudice, but I feel that as an educational experience the classroom has to exclude their opinions, authentic as they are. Of course a teacher can always introduce an example simply to disagree with it, but in general I think one does have to consider with authentic materials whether the actual content is acceptable educationally or linguistically, as one would do for any other type of material.