Travelling between cities in Malaysia and the rest of south east Asia is very cheap and easy by bus.

Between major cities they operate on an hourly and sometimes even half-hourly basis. Besides the national bus carrier (Transnasional) you have many private bus companies. Have a look at Journey Malaysia has a fully comprehensive listing http://www.journeymalaysia.com/bus.htm or ExpressBusMalaysia.com for information. The conditions of all roads and of most buses are good / very good comparable to some long distance coaches around the world.

Tickets can be bought at various counters at the bus stations and are extremely cheap compared to Western standards (between 15 and 40 Ringgit). Price difference between companies is usually only a few Ringgit. 

Most buses going to / coming from northern locations leave from Pudu Raya whilst  those to / from the south will by from Bersepadu Selatan (TBS) in Bandar Tasik Selatan.  For easterly directions the bus station is at Hentian Putra or Hentian Pekeliling.  However different operators will vary, so check with the carrier before setting off to the bus station. 

At almost every bus station in Malaysia you will be approached by "runners" trying to make you buy a ticket from a company which pays them a commission. Don't worry, they make a lot of noise but are otherwise harmless. If you tell them you already have a ticket they will leave you alone. Try to have your trip/bus company figured out beforehand and just go to the appropriate counter. As mentioned, there is not much price difference between companies. Find out which bus leaves first , how many stops it makes and how long these last.

Tip: be sure to take a jacket or sweater on board because the air conditioning can sometimes be very cold! And yes, like everywhere else in the world: watch out for pick pockets or other scams.