First ask yourself, are you planning to have a debate or a discussion? In a debate there will usually be a winner. In a discussion there will be a an exchange of views but there is no intention for one ‘side’ to defeat another by argument. Both possibilities can be interesting and useful but my advice is never to be drawn into a debate if a discussion will do.
Next decide if the question at issue is actually resolvable by argument. Clearly it is pointless to debate matters of religious faith. Remember too that de gustibus non disputandum (there is no disputing about tastes). I can’t prove to my family that they ought to like eating black pudding, although I might be able to prove to them that it is a wholesome food. It would probably be unfair to enter into a debate as to whether nuclear power stations should be abandoned unless they were proven to be cheap and safe, if you would wish them to be abandoned even if they were cheap and safe.
Make sure that the argument is one that you really want to win. This is extremely important in the workplace or the home. In families less civilised than mine it would be fatally easy to win an argument by superior logic and knowledge and then spend weeks in the spare bedroom, at least metaphorically. Even one’s friends may claim that one is a clever, rational, articulate person who abuses his way with words to appear to prove something that cannot be proved. It may just be simpler to lose.
Know the limits of your argument and don’t try to prove more than is strictly necessary to win your case. If, for example, you are trying to prove that the Labour Party should be elected at the next General Election (fairly easy) don’t get drawn into arguing that there should always be a Labour government (much more difficult).
Prepare. Understand all the details of your argument and if possible your opponent’s arguments. State clearly at the beginning what you intend to prove and, if there is any possibility of confusion, what you are not trying to prove. Tell the truth. To tell less that the whole truth is an extremely risky. To be caught out in a palpable untruth is catastrophic to your case.
Establish your credibility but don’t attempt to win by prestige. If debating a medical matter it is reasonable for me to admit to being a doctor but it would be quite unreasonable for me to attempt to influence an audience by claiming that ‘only doctors can expect to understand these matters’. If your argument has weaknesses admit them at the beginning. It is usually better for the audience to hear them from you rather than your opponent.
Tell a story or paint a picture if possible. If you are arguing against capital punishment tell the life story of someone executed by mistake or unjustly. I am a little unhappy about this suggestion since individual experience can be misleading and ‘hard cases make bad law’. Perhaps it all depends on how keen you are to win. If someone’s life or liberty were to depend on a successful outcome it is probably legitimate. Also if you opponent believes that some course of action is always right it is sufficient to demonstrate a single incidence of it being wrong.
Logic is extremely effective; it can be very helpful if you can pick up logical inconsistencies in your opponent’s case, and draw attention to them, but avoid sarcasm, ridicule and ad hominem (personal) arguments. Use humour like salt.