Assuming you are in a boffer larp.. I have to agree with the people who have stated that weight is your biggest enemy.
Most of the shields I have seen in LARPS were made out of plywood with the edges covered by pipe insulation and held on by duct tape. The shield could then either be painted or covered in a cloth with an appropriate design/pattern. The one thing to worry about is to make sure that the attachments for the straps and handle are well covered and will be safe to run around with. If the handle is bolted on, for example, you will want the bolt to come in from the front of the shield (to make it safe in that direction) and you should cut off the end of the bolt on the back of the shield and file down as smooth as possible (so you don't hurt yourself).
My favorite design used a boogie board instead of plywood. You can easily cut/shape the shield however you want and it is very safe. Cover the back with duct tape (preferably black or something that matches with the cover to help disguise what it started out as) and add a cloth cover (tied on in back). In this case, the boogie board is strong enough that you can cut a few slits through the shield and just thread your straps through without having to use any hardware. I've seen a few cases of people trying to use regular closed cell foam, but the shields got broken rather quickly.
One other thing to consider is the shape of the shield. Assuming your are in a system that doesn't allow head shots and where all other parts of the body are equal, you want to make the shield longer than you might otherwise think to help protect your legs, which become a prime target. Often, LARPS will have limits on the maximum volume and dimensions of the shield. Within these limits, make the shield as big as possible.