The nusach which is added to the Kedusha of Shabbos is not part of the essential Kedusha. Some hold that “L’umossom”, “Uv’divray…” and even “Yimloch…” are not part of the essential Kedusha. At these times you are allowed to think divray Torah. (MB 1)
The Shulchan Aruch and the achronim, including the Gr”a, all hold to be silent and listen to the chazzan (your shaliach) say the introductory “Nekadaysh…”. However, the minhag is that the k’hal says it also. (MB 2)
The chazzan should never drag out the end of his beracha – the k’hal might begin to respond too soon. This applies to anything that will elicit a response. (MB 2)
Answer “Kadosh Kadosh…” together with the minyan. If you answer while the chazzan is still saying “v’kara zeh el zeh…” you must repeat the response correctly. Do not answer after the k’hal finishes “Kadosh…” either. Therefore, if you are still saying a piut, interrupt it to answer Kedusha with the tzibbur. (MB 3)
If the chazzan or anyone else begins “Kadosh Kadosh…” late, it is still considered to be b’tzibbur as long as the tzibbur didn’t yet finish saying that possuk. (BeH “ella”)
It is good to keep your feet together as you recite the Kedusha with the chazzan. (ShA 2)
You must have extra kavona to be mekadaysh Hashem. Think about fulfilling the possuk “Venikdashti b’soch B’nay Yisroel”. In this merit, Hashem will rest kedusha upon you from above. (MB 4)
Lift up your open eyes to the heavens during Kedusha. Raise your heels and body upwards, as you recite “Kadosh… Boruch”… and “Yimloch…” (Rama 2 – MB 5,6,7)
Pause briefly between the words “shemakdishim” and “osso”. (MB4)
No talking in the middle of Kedusha until the omayn of Hokayl Hakadosh. (Rama 2 – MB 9)
If, in the middle of Kedusha, you hear another minyan answering Kaddish, do not answer Kaddish until you finish the possuk of Kadosh… (MB 8)
You must respond to Kedusha in shul even if you already said it at an earlier time. (Rama 2)