A very brief post recommending teaching specific versus generalized mands can be found here: http://goo.gl/UIiI4H along with references to a few studied that support this opinion. We need all the research we can have on teaching mands due to the extreme importance to people with autism and language impairments.
And we need practitioners to truly understand what constitutes a mand and be able to assess motivation and current controlling variables when helping an individual to learn to mand. Mand training is too often poorly done and it is both because to be done well, it takes a practitioner who is very fluent in verbal operants and because it is not always easy to assess motivation and control all the contingencies necessary to establish the mand repertoire, especially with individuals with very limited strong motivation for items controlled by others, and with long histories of absent, weak, or aberrant behavior functioning as mands.
This article by Mark Sundberg discusses 30 points about motivation from Skinner’s book, Verbal Behavior: http://goo.gl/oDg1yK.