Here is one of the great supplications to be said in the beginning of the prayer:
It was narrated that Abu Hurayrah said: When the Messenger of Allaah (peace and blessings of Allaah be upon him) started to pray, he would remain silent for a while. I said: “May my father and mother be sacrificed for you, O Messenger of Allaah. What do you say when you are silent between the takbeer and recitation?” He said: “I say: Allaahumma baa’id bayni wa bayna khataayaaya kama baa’adta bayna al-mashriqi wa’l-maghrib. Allaahumma naqqini min khataayaaya kama yunaqqa al-thawb al-abyad min al-danas. Allaahumma ighsilni min khataayaaya bi’l-thalji wa’l-maa’i wa’l-barad (O Allaah, put a great distance between me and my sins, as great as the distance You have made between the East and the West. O Allaah, cleanse me of sin as a white garment is cleansed from filth. O Allaah, wash away my sins with snow and water and hail).”
Narrated by al-Bukhaari, 711; Muslim, 598.