By ship
It is possible to take a boat along the river, all the way to/from Huay Xai, but there is no scheduled service and only occasional cargo ships, so odds are high that you'll have to charter. From Luang Namtha, Boat Landing Guest House/Green Discovery can try to arrange boats, at an estimated but highly variable price of US$170-400 for a boat that can handle 4 to perhaps 10 passengers. The trip takes two days and requires overnighting in a village along the day included, but take some backup food if the village dinner is not to your standard. In the dry season after November, the northern parts of the river towards Luang Namtha may not be navigable.
By plane
Luang Namtha Airport IATA: LXG has Lao Airlines flights to/from Vientiane 3x/week 1 hour, full fare US$120, Mon Tue Wed, 2011. Since 2009, there are no longer any services to Huay Xai or Luang Prabang, but the airport was renovated in 2008, with a 1600m paved runway, and there are distant plans to field international flights. Shared tuk-tuks wait outside for arriving flights and will take you to anywhere in town for 10,000 kip.