Rarely a problem at the beginning of a course, decreasing motivation is a phenomenon which can set in ( at the worst) as from lesson 2! Why?
A student has to feel he is getting somewhere, that he is making progress and that what he has learned is useful. Demotivation is a phenomenon perhaps most difficult to attack with advanced students. Here it can stem, quite simply from an inability to perceive progress. It is easy for a student who started his lesson with no knowledge of a language to see his progress; the half dozen sentences he is capable of using after an hours lesson seems enormous compared to the total absence of knowledge he had before. To a student with a substantial knowledge of the language the same amount of real progress seems infinitesimal compared with his already existing knowledge.
This can be aggravated by the fact that teachers themselves, with admirable motives, range far and wide in their search for appropriate materials for advanced students. Lessons may be based on newspaper articles, one off exercises taken from grammar books, sound recordings etc. This is all very well as far as it goes but a certain amount of perception of progress comes from a student being able to riffle through the pages hes covered in a book and to say to himself "Wow. Ive done all that.". Thus it pays to give a certain amount of thought to the advisability of abandoning a set programme and doing a one off lesson which will reduce the apparent rate of progress.
If a variety of material is being used frequently, insist that the student maintain a file with all the material, exercises, homework, etc. which has been chosen (If you dont it will all get dispersed and lost in a surprisingly short time). Such a file is obviously useful for revision and reference use and, at the same time it will remind the student of all the work which has been accomplished.
A motivating course for advanced students can often be achieved by limiting its aims. 50 hours work may be spread out over all the things that language use involves: the basic abilities of listening comprehension, speaking ability, reading and writing, but also the finer tuned skills, using the telephone, addressing an audience, negotiating, socialising, etc.
Instead of doing a general course which will have results which can be schematised in graph 1, (3% increase in ability in 4 domains) you can do a specialised course which will have the results schematised in graph 2, (12% increase but in only one domain). Students may well prefer the latter because the difference in skill level, in one domain is perceptibly enhanced in relation to the goal whereas, in the first case, the increase is hardly perceptible at all because it is spread out.
At all levels it is important for a student to know what he has learned. This is not always as obvious as it seems to the teacher. It is often worth taking the time at the end of a lesson to recapitulate and to point out to a student the various things which have been covered. (And if the teacher finds he cant do this, it is probably a sign that the lesson hasnt really been up to scratch and was probably largely chit-chat!). It is usually best to define material covered in functional terms. " We have practised the Present Perfect and used several new verb-preposition combinations. " is unlikely to generate enthusiasm in any but the most academically oriented student. " Now you know how to go into a bank, ask about exchange rates and get your travellerscheques cashed " is a more satisfactory summary for the average student.