The majority of tourists arrive by plane, all but a fraction through Bangkok's international airport. Chiang Mai, Phuket and Hat Yai airports also have immigration facilities and a small number of scheduled international flight arrivals, mostly from neighboring countries such as Singapore or China.
You can travel by train from Singapore, or Kuala Lumpur or Butterworth in Malaysia, crossing the border near Hat Yai with Bangkok as its terminus. Eastern & Oriental Express operates a luxury service from Singapore to Bangkok. It's a leisurely journey with several stops along the way.
There are road crossings from Malaysia in the south at Songkhla, Yala or Narathiwat. Crossings from Burma are frequently closed due to internal conflicts near the borders, and are often open only to nationals of Burma and Thailand. You can cross the one bridge to Laos at Nong Khai in the northeast. The main crossing to Cambodia is at Aranyaprathet.
Cruising has increased in popularity in Asia, as it has elsewhere. Bangkok, Pattaya and Phuket are now stops on major cruise lines.