Say 100 times a day -

أَسْتَغْفِرُ اللَّهَ وَأَتُوبُ إِلَيْهِ

Astaghfirullaaha wa 'atoobu 'ilayhi.

I seek Allah's forgiveness and I turn to Him in repentance.

Reference: Bukhari: 6307, Muslim: 2702