KOH PHANGAN TRANSPORTATION
The closest airport is Ko Samui (USM) which has frequent flights from Bangkok and Phuket, daily flights from U-Tapao and Singapore, and several direct flights each week from Chiang Mai and Kuala Lumpur. Transportation to the ferry dock is easy to find at the airport. Ferries depart several times a day with the last one around dusk.
The next nearest airport is Surat Thani (URT) on the mainland.
From Ko Samui: There are at least 3 ferries a day from Ko Samui's "Big Buddha" pier directly to Haad Rin. Ferries also leave from Nathon and Maenam piers to Thong Sala several times a day.
By bus and boat
The best way in by bus is by Governmnent Bus to the Na Dan ferry piers: these are the most direct, quickest, reliable, safest, and hassle-free services. Tickets for these services can be bought at Government Bus Terminals.
Buses also arrive in Surat Thani, capital of Surat Thani Province, from where passengers are inexorably swept up in the machine that will, several buses and at least one ferry later, spit you out on the rusty pier of Thong Sala.
Buses originating from Khao San Road (or others with travel agent sold tickets) are notorious for thefts from passenger luggage and should be avoided at all costs. Under no circumstances should passengers on Khao San Road buses leave valuables in bags that will go in the luggage stowage areas, even if the bags can be locked; consider it inevitable that every bag will be opened while the bus is in motion.
By train, boat and bus
Overnight train from Bangkok is an interesting option. Trains arrive in Surat Thani or Chumphon, and from there you can transfer by bus and then boat. Combined train-bus-boat tickets can be bought direct from the official Advance Booking Counter at Hualamphong station in Bangkok, although sometimes train get late, and your boat is already gone and you have to pay extra for the next boat. Thus joint ticket is not the best choice.
In theory, train tickets may be reserved up to 60 days in advance and paid for by email. In reality, the Thai authorities are lax in returning emails and/or will give the runaround, or flat-out refuse to reserve seats for non-Thais during peak travel periods (December - January and Songkran holiday in April). If your heart is set on going by train, start early, be persistent, and have a backup plan to go by bus or plane.