http://revach.net/article.php?id=1607

Section:  Avodah   Category: Shmoneh Esrei
Part 11 Shmoneh Esrei: Bareich Aleinu - Stop Crime, Pray For Parnassa

One should daven for his sustenance, because one who eats before davening for his food it is considered corrupt bread ("lechem megoal"). Therefore it is fitting at least at shacharis to beg to Hashem that He provide you with your portion of bread in a permitted way (b'heter v'lo b'isur). Because if there is anything forbidden in the food it contaminates the whole body and there is no 'food' for the soul. But, if the food is entirely permitted, there is in it 'food' for the soul (see book: Shaarei Kedusha 1:1). Therefore one should daven with a lot of kavana that his food comes from the side of purity and this is the meaning of "v'sabenu mituvecha".

And there is good reason to daven that HaKadosh Baruch Hu should provide for the poor, and that He should give to each his needs so that poor Jews will not depend on others. In our many sins, the majority of sins of the Jewish people stems from lack of livelihood (parnasa). If people had proper parnasa, they would not sway from the straight path at all.

The main sustenance is in the morning. Then HaKadosh Baruch Hu is full of mercy for Yisroel in seeing the nations turning to their gods and Yisroel serving Hashem with full hearts. Then he sustains and provides for us. In truth, according to strict justice, one who is not completely meritorious does not deserve food. The main sustenance (for almost everyone) comes from mercy and not from justice.

This is a short, practical version of Ya'aros Devash 1:1 which Rabbi Shlomo Wolbe zt"l condensed in Alei Shur pg.123. The original contains all kinds of fascinating mystical insights. The interested reader is referred to Ya'aros Devash 1:1, starting about half-way through.