Numerous buses leave from Bangkok's Southern Bus Terminal, generally in the late afternoon and evening. The trip takes from 12 to 14 hours, depending on the bus and road conditions. You'll need to go to either a travel agent or to the bus station to check exact times and purchase tickets in advance, especially for VIP buses. Costs run from B550 for a large 32-seat air-conditioned bus to B1,300 for 24 seat air-conditioned VIP bus. Most long-haul VIP buses travel overnight, but day trips are recommended, as Thailand's highways grow even more dangerous at night.
There are buses from Phuket to almost every major destination in southern Thailand. This includes, but is not limited to: Surat Thani, Krabi, Trang, Hat Yai, Satun, Phang Nga, and the ferry crossing to Koh Samui. You can check departure times at your hotel or the centrally located bus station just east of Montri Road, two blocks north of Phang Nga Road in Phuket Town.