What are the etiquette and supplication before relations with one's wife?

It is recommended to recite the supplication taught by the Prophet ﷺ, to begin with affection, and to maintain cleanliness and privacy.
The supplication: Bismillah, Allahumma jannibnasy-syaitan wa jannibisy-syaitana ma razaqtana (In the name of Allah, O Allah, keep Satan away from us and keep Satan away from what You provide us). Other etiquette includes a good intention to preserve chastity and to have offspring, being affectionate first, considering your spouse's feelings, being in a private place, and performing wudu' if you wish to repeat. See Marital etiquette.