Is it mandatory to praise Allaah and send salutation upon the Prophet (salla Allaahu 'alayhi was-sallam) each time before making du'aa in order for it (the du'aa) to be accepted? And, does wearing najis clothing (whether because of the presence of physical impurity or the clothes are inherently impure) affect the acceptance of du'aa"? May Allaah reward you all.

All perfect praise be to Allah, The Lord of the Worlds. I testify that none is worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

Part of the etiquette of supplication is to start with praising Allah, The Exalted, and exalt the mention of the Prophet . This is one of the means to have supplications answered. However, it is not a condition for the acceptance of the supplication and for answering it. If someone supplicated without praising Allah, The Exalted, it is not wrong and his supplication is hoped to be answered by the favor of Allah.

Another etiquette for supplicating and reciting Thikr is maintaining the ritual purity of the supplicant’s body and clothes. However, if his clothes are stained with impurity, this does not prevent him from reciting Thikr and supplicating Allah, and it does not hinder the acceptance of his supplication. But he should be aware that if he intends to pray, he must purify his clothes because the ritual purity of the clothes is a condition for the validity of the prayer.

Allah knows best.