I first repented of my sins - my wrong way of living, when I first truly turned to the lord, wanting Him to be part of my life. I still occasionally think or desire what I know to be wrong, and on these occasions I apoligise to God and get back on track. This truly wanting God in my life did change me because I rarely think of worldly things as I used to.
Although saved by Grace through Faith, we still sin and need to repent - turn back around, when we do.
1Jn 1:8 If we say that we have no sin, we deceive ourselves, and the truth is not in us.
1Jn 1:9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
So we ocassionally sin, then we confess and let go of that sin, and are in a right relationship with God again, I believe.