In praying, in trying to approach God with thoughts and words, the essential factor is nullification of self.
Especially when one recites the Shema prayer, thoughts should be concentrated on the Divine oneness, not as contrasted to duality or multiplicity, but as His all-inclusive unity, in the sense that nothing else exists.
The thought of the Divine Unity is, therefore, the core of all contemplation.
–Rabbi Adin Steinsaltz
From The Long Shorter Way, p. 222, by Rabbi Adin Steinsaltz