The one time I was on a jury the judge specifically instructed the jurors to use our judgement in determining the credibility of anyone testifying. I don't think there's any way around it--everyone knows that perjury is a given on one side or the other.
Jurors are still people, emotional judgements will get involved one way or the other.
The fact that jury trials are driven by emotion instead of fact was driven home for me by the parachute murder trial, where there was only circumstantial evidence, yet a 30 year conviction was achieved mostly based on the odd demeanor of the accused: http://en.wikipedia.org/wiki/Parachute_Murder