The Shulhan Aruch writes (Orah Haim 220) that one should arise early on Friday morning to begin making preparations for Shabbat. The Mishna Berura (Rav Yisrael Meir Kagan, 1839-1933), in Se’if Katan 2 (listen to audio recording for precise citation), explained that it is preferable to make one’s purchases for Shabbat on Friday, as opposed to Thursday or earlier during the week, because on Friday it is clearer that these preparations are being done specifically in honor of Shabbat.
Furthermore, the Mishna Berura comments that when one purchases an item for Shabbat, he should verbally declare that this is done "Li’chbod Shabbat Kodesh" – in honor of Shabbat. The Mishna Berura explains that one’s words have significant power to enhance the level of Kedusha of the action, and it is thus proper to make a verbal declaration when preparing for Shabbat stating that these activities are being done specifically for the honor of Shabbat. This is also mentioned by the Hida (Rav Haim Yosef David Azulai, 1724-1807), in his work Moreh Be’esba (4:139).
Moreover, it is mentioned in several books that even on Shabbat itself, when one eats, drinks and involves himself in other enjoyable activities in honor of Shabbat, he should verbally declare that he is performing these activities "Li’chbod Shabbat Kodesh." The Midrash relates that Haman, in describing the empire’s Jewish population to Ahashverosh, told the king that the Jews always say, "Today is Shabbat; today is Shabbat." One of the commentators explained this to mean that on Shabbat, the Jews were constantly stating that they are performing their activities in honor of Shabbat. This Midrash thus serves as an allusion to the concept of verbally declaring on Shabbat that he eats, drinks, and performs other activities in honor of Shabbat. Similarly, the prophet commands, "Ve’karata Le’Shabbat Oneg" – "You shall declare Shabbat a day of delight," indicating that one should verbally declare his intent to give honor to Shabbat through his enjoyment.
The Zohar comments, "Kol Bircha’an De’l’ela Ve’tata Be’yoma Shebia’ah Talyan" – "All blessings of above and below depend upon the Shabbat day." It has been noted that the first letters of this passage spell the words "Kebod Shabbat," alluding to the fact that we earn the special blessings of Shabbat by according the day proper honor and respect.
Summary: It is proper when preparing for Shabbat on Friday to verbally state that the activity is being done in honor of Shabbat. Likewise, on Shabbat itself, one should make a verbal declaration to this effect when he eats and engages in other forms of enjoyment.