"After sin immediately say in your heart the Psalm, 'Have mercy upon me, O God,' and say the whole Psalm in and from your heart. If it does not take effect the first time, try again : only say it the second time more heartily, more feelingly, and then salvation will speedily shine into your soul."
- Saint John of Kronstadt
Reference: http://www.archive.org/details/mylifeinchrist00sergiala