In Jewish weddings here in the western tradition, the breaking of the glass is often done by the groom at the conclusion of the ceremony. He grinds the class beneath his right foot and the guests typically shout, "Mazel Tov!" The origin and the meaning of the ritual are very difficult to pin down as there have been a number of stories circulating about it for centuries. For example, an article at Chabad.org which references these difficulties in detail says, "From the Talmud it would appear that breaking the glass served to engender sobriety and balanced behavior,..." and discusses more contemporary reinterpretations of the custom.
At Italian weddings, the couple both stomp on the glass together and attempt to grind it into as many pieces as they can. The ritual is usually done at the conclusion of the reception rather than the ceremony, but can certainly be used to conclude the ceremony if the couple wishes to do so. The number of glass shards is said to represent the number of happy years the couple will have together, so clearly they want to spend a little time crushing it as thoroughly as possible.
I recently officiated a wedding for a couple at the Tides Collocated Club who concluded their wedding ceremony with a breaking of the glass. After the Declaration of Marriage and their kiss, we simply placed a silk bag containing a wine glass on the ground in front of them and I said the following words:
Ladies and Gentleman, (Bride and Groom) would like to honor (Groom's) Italian heritage with a beloved wedding tradition. With the breaking of the glass we look towards their future. While they break the glass, think of the gifts they bring to one another and the beautiful potential that is their shared life together. Consider how you can encourage them and help guide and support them in being steadfast in the promises they have made. Make a wish or just think a special thought for their union.