Hi Jeanry,
it might help if I knew if we were discussing a center held shield or a strapped heater type shield. However, Casper Bradack has written a nice article on using the sword and shield that may be found here.
http://www.arma-ogden.org/content/view/9/30/ Basicly, the way I currently see it (subject to revival as I experiment and read), there are two primary ways to hold your shield, "open" or "closed." "Open" is what I use for a 1 on 1 fight (though I will close it to cut off a line of attack as the fight warrents), I use the "closed" position for shield wall type stuff and will "open" up when the lines close.
The fighter on the right is standing in a position that I would call "closed" with his shield, the fighter on the left would be hard to determine due to his twisted torso.
The fighter on the right with the oval shield is in a position that I would term "open."
The fighter in the center of this image with the orange shield is the "closed" position.
http://www.thearma.org/Manuals/NewManuals/DiGrassi/03001085.jpg From Digrassi, a good image of what I would consider to be a "open" position with the shield.
Be sure to look at your 16th century Italian Rotella manuals, Digrassi, Morrazo, Capo Ferro, etc.
Some have argued for a 3rd shield position where the shield is on the right side of the body with the edge towards the opponent, It may be seen in some period illustrations, and is used with the large duelling shields. I feel that the period illustrations are done that way in order to show the coat of arms rather than a shield position and that duelling shields are an entity unto themselves. My current opinion is that if it is used, it would be best done with a center held shield (such as a duelling shield or a viking round) rather than a heater. I personally do not use this guard since I do not like being that crossed up with a large shield. Also, it may be a temporary transition to be used as a displacement with a dueling shield to try and create and exploit a new opening. My opinion on this is still in a transitory state.
An possible image of this 3rd guard may be found here
http://www.thearma.org/Manuals/SoloT/0021.jpg The person on the right apears to have lost the fight while in the middle of this position to the person on the left who appears to be in the "open" position.
I find the sword positions of the 7 basic guards of the MS.I33 work fairly well with a large shield as well as a buckler. So do the binds, strikes and thrusts of the MS.I33. The larger the shield, the slower it will be, remeber that it may be faster to step around your shield than to move your shield, and that a shield is the largest knuckle duster I have ever used.
Good luck.
Brian Hunt.
GFS