The following is the same prayer being said during the Novena at the National Shrine of The Divine Mercy.

Prayer to Obtain Graces
Through the Intercession of Saint Faustina

O Jesus, who filled Saint Faustina with
profound veneration for Your boundless
Mercy, deign, if it be Your holy will, to grant
me, through her intercession, the grace for
which I fervently pray…

My sins render me unworthy of Your
Mercy, but be mindful of Saint Faustina's
spirit of sacrifice and self-denial, and reward
her virtue by granting the petition which,
with childlike trust, I present to You through
her intercession.

Our Father…, Hail Mary…, Glory Be…
Saint Faustina, pray for us.