Always dress to impress. If it is for a job interview then a business suit is a must. It's a lot easier to believe that your telling the truth about your abilities if you're dressed well. (If you walk in wearing shorts and a tank top they may not quite believe that you can type 80 words a minute even if you really can)
If you're just out in the regular world then wear decent, well kept clothes. People have a easier time believing a well dressed (and groomed) individual when they tell what they are capable of.
DON'T LIE. Most people aren't good at lying anyway. If you try and they see through you, they'll believe everything you say is a lie, or at least has the possibility of being a lie. Some people just like to make up a load of junk to make themselves look better. Don't.
Give specific examples of what you've done, whether it be in school or in the work place. It's really easy to say that you can do something. If you don't actually know how to do it though, it can be very difficult to give authentic knowledge and accounts of what you have not done.
The biggest thing is really to tell that potential employer, or whomever else, what you can do as though it's simple just simple fact.("Yes I can type 80 words a minute.") Never say anything like, "Most people I've seen can't do that." If you really can do what you say you can, just be modest about it and let your abilities speak for themselves.