This question originally appeared on Quora. 
By Marco Witzmann, German speaking Debating Champion '12, European Semi-Finalist (ESL) '11
The most important word in debating is because.
Whatever point you make in a debate, substantiate it by finishing your phrase using because, because only then do your points become clear and gain depth. And if your arguments have more depth than the ones your opponent has, you are likely to win a debate (when judged by an unbiased adjudicator).
A nice way to do this is using the SEXI-scheme: State, EXplain, Illustrate.
State.
Use clear wording in one short phrase, stating the point you are trying to make. Be aware that you may have more/different knowledge than your audience/opponent, so use simple words. Also make sure to start your argument with the statement, because by doing this, the other person is prepared to follow your explanation.
Making an argument is not about surprising the other person, it is about taking her by the hand and guiding her step by step from her point of view to yours.
The death-penalty should be abolished.
EXplain
Here the word because comes into play. Sadly many people don't debate in their daily lives on this level, but stick to only stating claims instead. To be a good debater, use reason and common sense to explain why your point is valid. Use small steps in your explanation to make your argument watertight and to leave little space to ambiguity or the lack of clarity. Mention even things that may seem obvious; more often than you think, they are not as obvious to other people as they are to you.
The death-penalty should be abolished, because the state should never kill innocent people if it can be avoided.
As a trial is always trying to reconstruct what happened, using evidence and testimonials, there is always (even a very small) chance that they got it wrong. By taking an accused person's life, you take away their chance and the one of their family, of benefiting from any new evidence which may prove their innocence in the future. This is why the state should always take the softest measure possible (life-sentence), because by doing this, it keeps the public safe, but recognizes at the same time the right to life of the innocently accused.
As trials are made in the present but the crimes happened in the past, there will never be a 100% certainty, and the state should therefore never become the murderer of a possibly innocent life.
Illustrate
Many times the explanation may be very logical, but it tends to abstract the topic. This is good, because especially moral and ethical questions always rely on bigger, complex principles. To give the other person the opportunity to confirm from their real-life experience the principles you just introduced, you should use examples to illustrate your point.
Choose them wisely, as they have to be speaking in your favour in many aspects. A great example can make you win debates, as the audience identifies with your arguments. If, on the other hand, you choose an unsuited illustration, your opponent can win against you in a debate, not by attacking your point directly, but by simply turning your own example against you.
There have been 140 accused and convicted people released from death row (according to the Death Penalty Information Center) in 26 states of the US since 1973. This shows that courts can many times not prove beyond any doubt the guilt of an accused person.
As these are only the few cases which have been discovered on time, we can only guess how many innocent people have been executed, without having found evidence proving their innocence, before their execution.
False accusations will always be found in a probabilistic court system, but the deaths of innocents does not have to be a part of it.
It may sound (almost too) simple, to just justify every claim you make, but trust me, the ones who are better at doing it than others win debating world championships.
As a training, start by trying to explain (seemingly) obvious claims like "Democracy is good," "Drugs are bad," and "Human dignity is important," and you will soon find that the most obvious claims are very difficult to explain.
When you are done with that, go through the arguments you made and find claims in them, which have to be explained again (like the ones I used, e.g.: "the state should never kill innocent people if it can be avoided," "False accusations will always be found in a probabilistic court system," ...).
You can already guess that your arguments will build a tree of explanations, which contain claims, which should be explained, using claims ... This is an infinite process. Following it to a certain level of analysis gives your arguments depth.
So every time you feel like you've already explained everything, just think of a 5-year-old child, who would keep asking you: "why?"