The bus (combi; S/5) from Cusco (Calle Puputi s/n, Cdra. 2; no phone) to Pisac takes 45 minutes to an hour. Colectivos drop passengers just across the river at the edge of town, a 3-block walk uphill from the main square (and market). From Pisac, buses return to Cusco and depart for other parts of the valley -- Yucay, Urubamba (both a half-hour journey), and Ollantaytambo (1 hr.) -- from the same spot.
Although a taxi to Pisac on your own costs about S/35, it is often possible to go by private car for as little as S/10 per person. Private cars congregate near the bus terminal and leave when they have three or four passengers; just get in and ask the price (everyone pays the same fare).
Boleto Turístico in the Valle Sagrado
The Cusco boleto turístico (tourist pass) is pretty much essential for visiting the Sacred Valley, in particular for the ruins of Pisac and Ollantaytambo, as well as the market and town of Chinchero. You can purchase it at any of those places if you haven't already bought it in Cusco before traveling to the Valley. You can purchase the partial ticket that just covers the Sacred Valley sites if you aren't planning to make use of the full ticket in Cusco.
You're best off getting information on Pisac and the entire Sacred Valley before leaving Cusco at the Tourist Information Office, Mantas 117-A, a block from the Plaza de Armas (tel. 084/263-176) or the iPéru office, Av. El Sol 103, Of. 102 (tel. 084/252-974). Cusco's South American Explorers Club (tel. 084/245-484) is also an excellent source of information, particularly on the Inca Trail and alternative treks, mountaineering, and white-water rafting in the valley. Inquire there about current conditions and updated transportation alternatives. Beyond that, the best sources of information are hotels.
Though you'll find an ATM on the main square, it's probably wise to exchange much of the money you'll need before leaving Cusco (especially if coming on Sun to the crowded market). There's a post office on the corner of Comercio and Intihuatana.