Shiva Abhishekam is usually performed to a Lingam representing his manifestation as a creator of good (by destroying evil).
In many temples, one finds a vessel hung over the Lingam called thaara paathra, that continuously drips water or other offerings onto the Lingam in deference to Shiva's desire for Abhisheka.
Some of the common offerings used for Shiva Abhisheka on Shiv yantra are:
4. Tender Coconut water
5. Vibhuti (holy ash)
6. Panchamruta (Curd based delicacy consisting of Panch(5) items: Milk, Sugar, Ghee (clarified butter), Honey, curd)
8. Sandalwood Paste
9. Ghee (Clarified butter)
11. Bael leaves
12. Flowers (AAKAMDA Flowers,DHATURA Flowers,BLUE LOTUS(or Pink Lotus/White Lotus)Flowers are auspicious.
Since Shiva is said to wear Nageshwara (Snake God) as an ornament around his neck, it is said that the fragrance of Aloe (which attracts snakes) is also a very holy item to be used for the worship of Shiva.
In contrast, it said that Lord Vishnu is Alankara Priyar (Desirous of ornamentation). Hence Vishnu Sthalas (places of worship of Lord Vishnu) have elaborately carved idols of Lord Vishnu with the alankaram (decoration ceremony) post the abhishekam, being a very elaborate ritual.