The best way to give Dawah is through good conduct and best moral behavior.
When people see a person who is quite different in a positive way, they will start wondering and questioning. This is effective both in private and in public, but mostly in private where people who are around you observe your behavior, whether at work, college or home.
When the companions of the Prophet Muhammad (peace be upon him) moved out of the Arabian Peninsula to spread Islam; they went to places where they did not speak the language. All they had to do is be themselves; being Muslims with best conduct. Eventually people were amazed by their moral behavior that they asked what religion they followed. Not being able to explain due to language constraint, all they told people is to simply follow their way and be like them; in other words, imitate them and they were eventually able to reach the hearts of millions.