I usually do 2 things when backing into a spot:
1) Use my side mirrors to get inside the lines with room to spare on each side.
2) Once I cant see the lines anymore, I go back and forth looking at the parking spot across from mine to make sure I am straight (usually they line up perfectly) and to the spots next to me to make sure I back in as far as I want.
It helps me back in perfectly all the time
