I agree that one can adapt some buckler techniques for a center held round shield. I am not so sure this is as applicable to a strapped heater shield, or a kite shield. I feel there are two basic positions for your shield, open and closed. The open position is the one you will use most for single combat. It is done by extending the arm holding your shield out before you (the arm may be fully extended or be held with a slightly bent elbow) with the face of the shield pointed to the outside. If you are left handed, this means the face of the shield would be to the left. I like to hold my sword in vom tag, pflug, tail, or a position similar to 6th ward in the I.33. The closed position has the shield held with the face towards your opponent. This is the position most likely used for mass combat, in a shield wall. This poisition can be used to transfer into for warding off a blow in single combat, but is not a good place to hang out. It is too easy for your opponent to pin your shield and control you with their shield. There is a potential third position, where you cross your arm accross your body and hold the shield on the other side of your body with the shield facing inside, or to the right if you hold your shield in your left hand. I have considered this to be a form of a displacement and haven't made up my mind on this one yet, if you use it, I would only transition through it for specific purposes and never hang out in it. This 3rd option works best with a center slung round. A shield, unlike a buckler, has a lot of mass, it is easier and quicker to step around your shield than it is to move your shield while you stay static. Interestingly enough, this is how Talhoffer, and the other masters that deal with them, handles those huge duelling shields. You may be able to extract some techniques out of these various duelling shield manuals. These 3 positions can also be found in a lot of pictures, statues, and carvings from the middle ages.
Also, one of our members Casper Bradock has written a really good article on using the heater shield. It can be found here.
http://www.arma-ogden.org/content/view/9/30/ hope this helps.
Brian Hunt
GFS