Evan Schaeffer has posted an interesting tip, reminder, on his Trial Practice Tips Weblog.  There are two requirements for success in exposing the liar.

First, you must be certain that you can establish that the witness has a "clear-cut motive to fabricate that the jury will understand";

Second,  you must be certain that you have at least one "clean substantive line of cross-examination" during which you can establish that the witness probably lied.

So for example I recently litigated a motion to suppress based on lack of voluntariness due to the effects of little sleep and alcohol.  The agents both testified in absolute terms that they did not smell alcohol on the client during their contacts with the client and the interrogation.  However, shortly after the interrogation the client was medically examined by a corpsman.  The corpsman was able to testify to smelling alcohol on the clients breath.  The agent’s motives to lie are fairly obvious in this situation and the existence of an independent witness gives you meat for the line of questions.  In a situation like that I will then argue the maxim "falsus in uno, falsus in omnibus."