make a positive thought to look at were you are going to reverse before doing so. get into the habit of looking earlier i.e. when you arrive somewhere think 'how will i turn round/get out? ' then when you go to leave check that nothing has changed since you looked on your way in.
my mate has had loads of prangs reversing vans and just recently passed his hgv. i told him to take that extra second or so to check and he'll save himself so much grief.
it's quicker to hop out, look for definate,then carry out a clean manouver than it is to hope it's ok, hit something and spend ages sorting it out.
as a proffesional driver of many years ican only say get in the habit of looking first.