Base your argument on commonly-held values. If you show that your conclusions are based on common assumptions, your argument will look correctly-aligned with something people tend to agree with. Help people relate to your argument so it appeals to their core and personal beliefs. Here is an example: "I believe nature is one of our most precious resources; with this in mind, we should consider whether deforestation betrays this belief."
Portray your argument as logical. Start with a major premise -- for example: "Organizations which are honest about their taxes have nothing to fear." Then, introduce a minor premise: "Organization X is reluctant for people to view its tax returns." Then introduce the conclusion: "So the question is, what does organization X fear people will find out?"
Use emotion to express your enthusiasm and belief in the argument. Be confident. Show your audience that you are emotionally moved by the subject. Be sincere. If you don't seem genuine, you may appear manipulative.
Avoid creating a logical fallacy. Check your facts. Don't make assumptions. For example, the "post hoc ergo propter hoc" fallacy refers to the assumption that because event B occurred after event A, event A caused event B to happen.