What are the constants and conditions of da'wah (invitation to Islam)?

The Prophet, peace and blessings be upon him, clarified the fundamentals and constants of da'wah (invitation to Islam) when he sent Mu'adh to Yemen, by stating that one should begin with Tawhid (monotheism) and then the other pillars of Islam according to their importance. If the invitees are Muslims, they should be called to correct the other pillars.

As for the conditions of da'wah, the most important are knowledge of what one is calling to, then the use of wisdom, good counsel, and patience. The da'i (caller) should also be an example by applying what he calls to, as Allah Almighty says: (Do you enjoin righteousness upon people while you yourselves forget it?).