Aisha, may Allah be pleased with her, said: I asked the Messenger of Allah: 'O Messenger of Allah, if I know what night is the night of Qadr, what should I say during it?' He said: 'Say: O Allah, You are pardoning and You love to pardon, so pardon me.' "(Ahmad, Ibn Majah, and Tirmidhi).
The transliteration of this Dua is "Allahumma innaka `afuwwun tuhibbul `afwa fa`fu `annee"
For more tips on how to make the best of the last 10 days of Ramadan click here, here, and here.