If you're playing a format involving cards printed earlier than the _Urza's Saga_ set, you may not reorder your graveyard. In this case, you need to be more careful when selecting an instant card at random, perhaps by using dice, writing the names of the instant cards on slips of paper and choosing one of them randomly, or carefully noting the existing graveyard order so you can reestablish it after performing the method suggested above, for example.