I know that this has been covered before but I just had occassion to retreat 3 Corps over 3 seperate bridges and I remembered advice someone had posted some time ago. So I thought it was worth sharing again with the addition of Screens. I used Tutorial 3 to illustrate the method.
I have found that the best way to retreat in any type of good order, is to give an Order to "Deploy (not Defend) in Checkerboard" to a line in you rear and change the Order to "Defend" when you arrive at the line. With this type of in depth unit protection the Corps will usually disengage from Combat. Some units in heavy Combat may be left behind but that's realistic and shields your main force withdrawal. Beware retreating an engaged Corp is a risky thing and may turn into a Rout.
There is also a Screen of the PBEM battle Retreat over the Bridges.