The only port of arrival by air is Wattay Airport in Vientiane. You can travel by Lao Aviation, Thai Airways International, Vietnam Airline, The Southern China Airline, Silk Air, Malaysian Airline and Regular flights are available to Bangkok, Phnom Penh, Ho Chi Minh City, Hanoi, Chiang Mai, Kunming, Kuala Lumpur and Singapore.

By river

To enter Laos via this border crossing, you take a ferry from Chiang Khong (20 baht, or U.S. 50 cents). First, in Chiang Khong, you must go to the customs window, which is right by the ferry, and get your departure stamp from Thailand. This is free. The ferries leave frequently throughout the day, basically whenever someone needs one. It is by longboat with a motor attached.  On the Laos side, you can get your Laos visa (up to 15 days--30 days visas take longer and cost more)

By air

The national airline of Laos is Lao Aviation (QV) which serves the international routes from Vientiane to Hanoi (Vietnam) Bangkok Phnom Penh (Cambodia) Chiang Mai (Thailand) Kunming (China) and Ho Chi Minh City (Vietnam). Thai International flies from Bangkok Vietnam Airlines flies from Hanoi and Royal Air Cambodge flies from Phnom Pen.

Arrival by Road

From Thailand
Road access is possible from neighboring countries. Between Thailand and Laos the border can be crossed at Houeixay in Bokeo province, at Mittaphab friendship bridge near Vientiane and at Dan Savannakhet in Savannakhet Province.

From China
It can be reached by road at Boten in Luang Namtha province.

From Vietnam
Border crossings between Laos and Vietnam have been set up at Lak Sao on the road No.8. Borikhamsay province and at Dan Savanh in Savannakhet province on the road No.9.

From Cambodia
Overland travel to/from Cambodia is conducted by river only, from Stung Streng, via the border town of Veunkham.  It's a bit tricky, and you can easily pay

Arrival by Train

Although there is currently no rail connection to Laos yet, an extension of the Bangkok-Nong Khai railway is planned and a line from Vientiane to Nong Khai is scheduled to start operating in near future.