Do not interrupt a short beracha, even for “Omayn y’hay sh’may rabba…” (MB3)
Whoever speaks even between Yishtabach and Yotzer has committed a transgression and is disqualified from fighting in a war. (ShA3)
During the Ten Days of Teshuva, writes the Ari z”l, say the chapter of “Mimamakim” between Yishtabach and Yotzer. (MB4)
Kaddish is never recited without either tefilla or pesukim of praise beforehand. (Rama3 – MB 8,9)
Kaddish D’rabbanan was instituted specifically to be said after aggadda. Therefore after a learning session that is not aggadda, be sure to recite the statement of Rabi Chananya ben Akashya or the like before saying the Kaddish. (MB9)
Don’t say the Kaddish unless there were ten men present during the learning even if only two or three were actually learning. (MB9)
Because Yishtabach ends a section of the daavening, you are allowed to interrupt for a mitzva purpose before beginning the new section of Yotzer. For example: If you didn’t have tallis or tefillin before then, you may put them on with a beracha as long as you’ll finish putting them on before the chazzan gets to “Omayn, Yehay shemay rabba…” (Rama3 – MB6,12)
If you won’t manage to put them on by then, wait until after Borchu to put them on without the berachos. After the tefilla, feel the tsitsis and recite the beracha. But for the tefillin, feel them and say the beracha at the earliest bayn haprakim – just before starting Ahava Rabba. (MB12,13)
If you had to answer Kaddish and Borchu while still saying Pesukay D’zimra, you have not entered into the catagory of b’emtza haperek. You can therefore still say the beracha on tallis and tefillin. (MB14)
The main intention of the chazzan when he says Borchu is that the congregation should begin reciting the beracha of Yotzer Ohr. (MB 13)