The Shulhan Aruch (Orah Haim 180:2; listen to audio recording for precise citation) writes that if a person does not leave some bread from his meal on the table during Birkat Ha知azon, he will not enjoy blessing. This comment is based upon a passage in the Zohar (Parashat Yitro). The Beracha that comes from Birkat Ha知azon needs some bread on which to be bestowed, and thus one should leave over some bread from the meal and leave it on the table, and not eat all the bread from the meal.
If a person did eat all the bread before Birkat Ha知azon, he should bring some other bread and put it on the table for Birkat Ha知azon. There is some discussion among the Halachic authorities as to whether in such a case one may bring a whole loaf. The Shulhan Aruch writes that after one has finished eating he should not place a full loaf on the table, as it was customary among idol-worshippers to place full loaves on the table to their gods. The Shulhan Aruch cites in this context the verse, "Ha弛rchim La暖ad Shulhan," which describes how the pagans would "set the table" for their deities. However, the Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) cites those who maintain that this applies only if there are already pieces of bread on the table. If one then brings a full loaf, it appears as though he is bringing bread for a pagan god. But if one has no bread on the table, he may bring a whole loaf to have bread on the table for Birkat Ha知azon. Others disagree, and forbid bringing a loaf on the table after the meal under all circumstances, even if there is no other bread on the table. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) thus writes that if one has no bread to put on the table for Birkat Ha知azon except for a full loaf, he should ensure to eat a small piece from the loaf so it will no longer be complete, and he may then place it on the table. This is the proper practice to follow in such a situation.
Summary: It is proper to leave some bread over from the meal and leave it on the table for Birkat Ha知azon. If no bread remains from the meal, one should place other bread on the table. It is improper to place a whole loaf on the table after a meal, and thus if all one has in a whole loaf, he should eat a small piece from it so it is no longer whole.