This page describes a system developed by the Hendon Police Driving School to help in this manoeuvre. <<download a PDF version>> Or the is a another technique below...
It's all done with Mirrors...

	1.	Pull up along beside car 'A', with a space of about one foot between you, and with your windscreen in line with the front of car 'A'.
	2.	If clear to do so, reverse in a straight line until your rear wheel has just passed the rear of car 'A'.
	3.	If still clear to do so, put full left lock on whilst still slowly reversing. (Steering wheel fully anticlockwise).
	4.	Reverse into the gap, until you can line up in your door mirror, the side of your car and the nearside lights on car 'B', then centre the steering wheel.
	5.	Continue reversing, in a straight line until the front of your car has cleared the rear of car 'A'.
	6.	While still moving slowly, apply full right lock, (the steering wheel fully clockwise), until adjustment is needed to park near and parallel to the kerb.
	7.	Centre the steering wheel and place the car into the centre of the gap.
Note: As with all manoeuvres never let the car travel too quickly, especially when turning the wheel. You should never turn the steering wheel whilst the car is not in motion as this can cause excessive wear to steering joints, bearings and tyres. Always be on the look out for other road users, including pedestrians. Be prepared to give way, even if this means that a manoeuvre has to be temporarily abandoned.
Or the Alternative version (thanks to Nick Reeks)
1.   Pull up along side the target car you are going to reverse behind. Stop so that the right rear corner is hidden by the ‘C’ pillar. 2.   When you start to reverse immediately steer about ¾ to 1 turn to the left. 3.   As the passenger door mirror is nearly level with the corner of the car straighten up. 4.   As the left front corner of your car clears the corner of the target car steer about ¾ to 1 turn to the right. 5.   As your car comes parallel to the kerb, rapidly straighten up. Parking brake on, select neutral and check. Switch off. Job Done.