Tfilla is accepted at a time of good will - עת רצון
Tehillim creates this עת רצון and enables our tefillos to be accepted. That is why whenever we are in trouble we say tehillim - that is what creates an עת רצון.
There is no greater עת רצון than hoshana rabba. That is why the ushpizin of hoshana rabba is Dovid Hamelech - the author of Tehillim and Sweet Singer Of Israel.
YIDDEN - SAY A LOT OF TEHILLIM TONIGHT AND THEN DAVEN HARD!!!
:)
[Maran in the Ma'amarei Pachad Yitzchak]