Competitive high-school debate demands a resolution which creates opposing points of view and a neutral judge. The judge is like a balance scale. The weight of the debater's arguments will tip the balance arm of the scale to one side or the other and the weight which tips the scale is the force of the debater's persuasive skill. At the end of the round it doesn't matter what the truth may be with respect to the resolution nor does it matter what the opinions of the audience or debaters may be. All that matters is which side produced enough of an influence to tilt the balance of the judge's opinion. In fact, the judge doesn't have to agree with a side for it to win. The judge only needs to think the one side tilted the balance to their favor. Even if he thinks both sides were horrible debaters, all the matters is which side was the least horrible. It is important to realize, however, that even though the judge is a "clean slate" or neutral in opinion, the judge may still possess prior knowledge of the issue or data which support a given position. When opposing information is given which violates or challenges the judge's prior knowledge, the judge will resist and at worst, reject the claim until sufficient persuasive weight is applied. So while a judge may initially be devoid of opinion as to which side will prevail, there is nevertheless, a tendency to reject or accept arguments because of the judge's prior experience and knowledge. It is very important to understand that reality.
The first chance debaters get to begin the process of tilting the balance is during the opening speeches, also called the constructive speeches. The constructives, as they are called, are pre-written and are supposed to supply the claims, grounds and reasons for why a particular point of view should be favored. Since the constructives are written in advance the persuasive power is supplied in the force of the language and how it's presented. Debaters who study persuasion learn of three mode of persuasion; ethos, pathos and logos and the format of persuasive speeches provide the means to employ each mode. Ethos encapsulates the credibility of both the speaker and his case. Pathos conditions the mental and emotional state of the listener. Logos defines the systematic series of arguments which drive the conclusion.
Debaters learn very early that claims are usually backed by data and data should be supplied by credible sources. The credibility of the data and evidence is essential to establishing the proper ethos of persuasion. One should also consider the ethos of the debater as well. A debater who knows what he or she is talking about is more persuasive than one who does not quite get the nuances or details of a topic. Therefore, its very important for debaters to present themselves as authorities by virtue of their exhaustive study of the topic and preparation of the case. Much of this personal ethos will be projected in the delivery and presentation of the speech itself. Is it delivered with confidence and enthusiasm? Are the words and names pronounced properly? is the content of the speech well known? Personally, I see no problem explicitly making remarks which establish one's credibility, as long as its done subtly. For example, "...after an exhaustive search of the evidence, we have come to the conclusion..." This can invoke a submissive response in the judge as long as claims are not subsequently made which violates the judge's prior knowledge.
Pathos/State of the Listener
I have heard some say pathos is passion and it is reflected in the passionate way in which a speech is delivered. Others say that it is an appeal to the emotion of the judge. Debaters must be cautious about making appeals to emotion. They are expected to win on evidence, logic and sound reasoning. One would hardly think that because we feel sorry for a group all courses of action to alleviate the suffering of the group are justified. Pathos deals with the mental and emotional state of the listener and yes, I think it can involve direct appeals to emotion on a certain level. The most overt way to utilize pathos is through the use of impact claims. It is very common, in fact, often necessary to carry the claims in the speech to some sort of impact statement. An impact statement tells the judge why a claim is important. It takes the form of "if such and such is done it leads to something bad" (a harm or negative impact) or "if such and such happens it results in something good" (an advantage or positive impact). These impacts are claimed consequences which evoke an emotional response in the experience of the judge. Positive impact statements can be very advantageous for the debater depending on how they are presented. For example, if the claim is made that a course of action can result in millions of people being saved from starvation, there is an implicit notion that millions of people must be on the verge of starvation in the status quo and so an emotional reaction, however subtle, may be triggered in the judge without the debater focusing on a direct emotional appeal of mass starvation. There are other, less common ways to utilize pathos including the use of metaphors or figurative language but care must be exercised in their application.
Logos/arguments leading to conclusion
Probably for most debaters, the logos mode of persuasion is most understood since basically it describes the well-reasoned arguments which eventually lead one to the desired conclusion. Logos defines the collection of claims and their associated evidence presented in a cogent and logical way. The goal is to build the case in such a way that upon hearing the evidence or reasoning leads one to no other possible conclusion than the one desired. This mode will be developed more fully in future postings.
Changingminds.org info on the three modes of persuasion
Durham Tech - Ethos Pathos Logos
Wikipedia - Modes of Persuasion