The etiquettes, manners and characteristics necessary for the du’aat (callers to Islaam) has been duly explained by Allah – the Mighty and Majestic – in many aayaat  and in many different places in the Noble Qur’an. And from those necessary etiquettes are:

FIRSTLY: SINCERITY
So it is obligatory upon the daa’ee (caller) to have ikhlaas (sincerity and purity of intentions and actions) for Allah – the Mighty and Majestic – neither desiring to show-off, nor desiring reputation, nor desiring the praises and the accolades of the people. Rather, the daa’ee should only call to Allah seeking the Face of Allah, as He – the One free from all imperfections – says:
“Say: This is my path, I do call to upon knowledge.” [Surah Yusuf (12):108]

And Allah – the Mighty and Majestic – said:
“And who is better in speech than one who calls to Allah.” [Surah Fussilat (41):33]

Thus it is a must to have ikhlaas and to call only to Allah – the Mighty and Majestic – and this is the most important ettiquette and the greatest quality; that you seek from your da’wah (call) the Face of Allah and the Home of the Hereafter.

SECONDLY: KNOWLEDGE
To call the people to upon ’ilm (knowledge), and not ignorance: “Say: This is my path, I do call to Allah upon knowledge.” [Surah Yusuf (12):108]

So knowledge is an essential obligation in calling to Allah. So beware of calling to Allah based upon ignorance, and beware of speaking without due knowledge. Indeed, ignorance destroys, it does not build; and it causes corruption, not reformation and correction. So – O worshipper of Allah – fear Allah and beware of speaking about Allah without due knowledge, and do not call to anything except after knowledge of it and having insight into what Allah and His Messenger (sallAllaahu ’alayhi wa sallam) have said.
So it is upon the student of knowledge and the daa’ee to have knowledge of, and insight into that which they call to, knowing its proofs and evidences.

THIRDLY: MILDNESS AND GENTLENESS
From the akhlaaq that it is necessary to have – O daa’ee – is to be mild and forbearing in your da’wah, and being gentle and patient in it, as were all the Messengers alayhimussalaatu was-salaam. Beware of being hasty, harsh and strict in your da’wah, rather be patient, mild and gentle. In this regard, some of the proofs have already preceeded, such as the saying of Allah – the Mighty and Majestic: “Call to the way of your Lord with wisdom and beautiful admonition, and argue with them in ways that are best.” [Surah Nahl (16):125]

And His – the Most Perfect’s – saying: “And by the Mercy of Allah you were able to deal gently with them. If you had been severe and harsh-hearted they would have broken away from about you.”