If you're staying for an extended period in KL or Malaysia, consider the prepaid MyRapid (www.myrapid.com.my) card, valid on Rapid KL buses, the monorail and the Ampang and Kelana Jaya LRT lines. It costs RM20 (including RM5 in credit) and can be bought at monorail and LRT stations. Just tap at the ticket gates or when you get on the bus and the correct fare will be deducted.
The Touch 'n Go card (www.touchngo.com.my) can be used on all public transport in the Klang Valley, at highway toll booths across Malaysia and at selected parking sites. The cards, which cost RM10.60 and can be reloaded with values from RM10 to RM500, can be purchased at KL Sentral and the central LRT stations KLCC, Masjid Jamek and Dang Wangi.