Or alternatively a logical order of argument: I think it amounts to the same thing. I don't know which I prefer.

My feeling is that "prelación" is being used here in the sense of prioritization or order of precedence in logical argument. That sense seems to me to fit the context, in that you would expect the recommendation here to refer to providing evidence and constructing a proper argument.

I wondered whether it might refer to establishing premises for one's conclusions: whether "una prelación" might mean an initial hypothesis or premiss. I still wonder whether that is the sense. It was suggested to me by the following use of "prelación" in a Venezuelan university program, where it means evidently means "prerequisite":

"SEMESTRE II
Descripción
DERECHO CIVIL PERSONAS
DERECHO ROMANO II
Prelación: DERECHO ROMANO I"
http://www.urbe.edu/Pensum/PensumView.do?idPensum=861

So I thought maybe "una prelación" might, by analogy, mean a prior basis for the argument. Actually I think the sense is very probably along those lines, but not precisely that; extrapolating too literally from the source just quoting seems to me speculative and unsafe.

More convincing, to me, is an analogy with uses of "una prelación" in legal argument, where it is to do with precedence and prioritization: discerning which arguments or pieces of evidence take precedence over which others. As here, for example:

"cuarto: relacionar, en forma sucinta y sintética, el material probatorio y su correspondiente valoración, conforme a una prelación: inicialmente las pruebas de cargo y después las de descargo [...]"
http://books.google.es/books?id=Z-rRokiVBC8C&pg=PA36&lpg=PA3...

I think that "los tiros van por ahí", as they say. In fact I wonder whether we could be more literal and use an expression like "prioritization" or "order of logical precendece", in the spirit of the following comment in the Wikipedia page on critical thinking:

"Critical thinking calls for the ability to:
Recognize problems, to find workable means for meeting those problems
Understand the importance of prioritization and order of precedence in problem solving
Gather and marshal pertinent (relevant) information [...]"
http://en.wikipedia.org/wiki/Critical_thinking#Importance

I would not rule it out: a possible translation could be something like paying due attention to logical order of precedence. However, this does sound somewhat forced to me, and I would finally recommend a freer translation, expressing the idea of priority by the word "order" or "ordered": following a proper order of priority meaning arguing things in a logical order of cause and effect.

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Note added at 4 hrs (2012-03-12 10:06:50 GMT)
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The context suggests to me that "prelación" refers to the arguments, rather than the evidence: "de acuerdo con una prelación" qualifies "expone su posición", not "evidencias". The idea of establishing an order of priority among the pieces of evidence presented -- discerning which are more important than which others -- makes good sense in itself, and I think that is included in the sense of "prelación", but "expone su posición de acuerdo con una prelación" seems to me to refer to the logical order of precedence of the propositions from which the argument is constructed, rather than the supporting evidence.

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Note added at 7 hrs (2012-03-12 13:11:15 GMT)
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On reflection, I think you could reduce it to: following a logical order. This is really the sense of "una prelación" in the Google Books reference above. I still feel that it refers to the order in which things are presented.