While the children are showing and telling I keep my card strips and sharpie nearby so I can write down what it is they are sharing.
After they finish we quickly discuss what letter they think the item starts with and then each child gets to place their card with the corresponding letter. It's been very successful so far! Although my current class is small I know larger classes would probably not be able to do this unless show and tell is narrowed down to only a few but since we're small everybody shares every week!