I have a tradition from my Master [the Baal Shem Tov] that whether Shavuot falls out on the [same] day on which our ancestors received the Torah, in the time when they sanctified the moon according to visual testimony,1 or on the seventh [of the month of Sivan] according to Rabbi Yossi,2 that during the reading of the Ten Commandments, one should concentrate on the Divine Name derived from the verse: "Open my eyes, that I may behold wondrous things in Your Torah" (Psalms 119:18) [interspersed with the Name Y-H-V-H]: (Siddur Rav Shabsai)

One must be extremely careful not to speak a single idle word from the recitation of the Tikun of Shavuot night 3 night until after the Kedusha prayer of Musaf.4 For then, all 24 Ornaments of the Bride ascend. (See Rashi on Exodus 31:18) These ornaments correspond to the 24 books of Scripture that comprise the main part of the Shavuot night Tikun, as well as the 24 possible combinations of G·d's holy Name Ado-nai that correspond to the sefira of malchut. By reciting the Tikun of Shavuos night, one clothes the Bride - the Divine Presence - in beautiful garments that make her fit for union with the King.

Thus, one should be very careful to purify their thoughts at least until this time. (Siddur Rav Shabsai)

[From Sefer Baal Shem Tov; translation and commentary by Rabbi Dr. Eliezer Shore; First posted on baalshemtov.com]