Here's a tip that I use all the time when
parallel parking.
More often than you think, you'll be parking out the front of a
shop with a big
shiny window... Well, as you're backing into you're spot, look in the window at the
reflection of your car and the car behind you... See what I mean! You can see exactly how close you are to the car behind ... so now you can
reverse until you are a few millimeters away and then straighten up with plenty of room in front.
This is especially useful when you are driving a friends car that you are not familiar with the
dimensions of.
Happy parking!