Our practice is to say "Amen" at the conclusion of the blessing of Yishtabah – the blessing that ends the morning Pesukeh De'zimra service. Some people mistakenly say "Amen" after the Hazan recites the words "U'mehulal Ba'tishbahot," but this is incorrect. "Amen" is recited at the very end of Yishtabah, after "Melech Kel Hai Ha'olamim." The reason why "Amen" is recited at the end of Yishtabah is that this Beracha concludes a "series" of Berachot, which began with the recitation of Baruch She'amar. Baruch She'amar is the introductory blessing of Pesukeh De'zimra, and Yishtabah is the concluding blessing of this section. Our custom is to recite "Amen" at the end of a Beracha which concludes a series of Berachot, and so we say "Amen" upon concluding Yishtabah. Incidentally, this is also the reason why the Beracha of Yishtabah does not begin with the words "Baruch Ata Hashem," like most Berachot do. There is a fundamental rule that a Beracha which follows another Beracha, as part of a series of Berachot, is covered by the introduction of "Baruch Ata Hashem" that opens the first Beracha, and so it does not require this introduction. Yishtabah is considered as though it follows Baruch She'amar, because everything in between – the chapters of Pesukeh De'zimra – are essentially the continuation of Baruch She'amar. We begin our praise of Hashem with the recitation of Baruch She'amar, and we then continue with the praises of Pesukeh De'zimra, concluding with Yishtabah. Therefore, Yishtabah does not begin with "Baruch Ata Hashem." Other examples of this rule include the Amida prayer, which consists of 19 Berachot, only the first of which begins with "Baruch Ata Hashem." All subsequent 18 Berachot are covered by the introduction of "Baruch Ata Hashem" that opens the first Beracha, and so they do not require this introduction. Similarly, among the blessings recited before and after Shema, only the first begins with Baruch Ata Hashem ("Yoser or" in the morning, "Asher Bi'dbaro" in the evening), as all the subsequent Berachot are the continuation, forming a series of blessings. Another example is the concluding Beracha of Hallel ("Yehalelucha"), which does not begin with "Baruch Ata Hashem" because it is the continuation of the introductory Beracha of Hallel.