Dawat by action and presenting a model to follow by mankind


لَقَدْ كَانَ لَكُمْ فِي رَسُولِ اللَّهِ أُسْوَةٌ حَسَنَةٌ لِمَنْ كَانَ يَرْجُو اللَّهَ وَالْيَوْمَ الآخِرَ وَذَكَرَ اللَّهَ كَثِيرًا (الأحزاب: 21

Laqad kana lakum fee rasooli Allahi uswatun hasanatunliman kana yarjoo Allaha waalyawmaal-akhira wathakara Allaha katheeran

There is indeed a good model for you in the Messenger of Allah - for the one who has hope in Allah and the Last Day, and remembers Allah profusely.

Dawah by action and Amaal Presenting a model is most effective way of Preaching. Da'wah generally takes place by means of speech However it also takes place to a greater extent by means of action. 
People accepted the invitation of Islam after they became attracted towards the good actions, noble qualities and humility of those who invited them. 
This was the manner of Rasulullah (Sallallahu Alaihi Wasallam).
 He was extremely kind and tolerant, even towards those who were rude. 
He would then invite them towards Islam and they would accept.