Short Answer:
From skin to skin contact.
Long Answer:
If a person has an active cold sore on their lip the herpes virus particles can be transferred to any part of the body that is kissed, including the mouth, face or the genitals. Keeping this in mind, it is very important to avoid kissing and oral sex during an active bout of cold sores.
It should also be made aware that the virus is considered to be contagious from the very first sign of an outbreak (including the tingling, itching stage) until the area is completely healed again. The cold sore virus can also spread at times when there are no visible symptoms due to what is known as Herpes Viral Shedding.
Herpes is most likely to be spread from the time these first symptoms are noticed until the area is all the way healed and the skin looks normal again. Contact with the infected area (including oral sex, kissing and touch) is very dangerous during this time.
It is also possible to transmit virus particles to another person through average objects such as a drinking glass, eating utensil, lip stick, lip gloss, toothbrush, a wash cloth, etc.