ATMs available in Lhasa, Shigatse and a couple of other towns. Credit cards can be used in Lhasa. Otherwise bring cash.
Tibet is one of those wonderful places where tipping is generally not done or asked for. If you go on a long organised trip out to Eastern or Western Tibet, however, your guide and driver will probably expect a tip at the end, assuming all went well. ¥30 to ¥40 per day for the guide and ¥20 per day for the driver is considered usual.