By ship
With the improvement of the roads, ferry services along the Mekong River from Kampong Cham no longer run. If you are feeling adventurous, you might find a rice-barge on its way north from Kampong Cham.
By road
From Kampong Cham to the south the road is currently in good repair. From Stung Treng to the north the road is sealed but has fallen into disrepair and the journey can be uncomfortable and slow; if headed into Laos, visas on arrival are now obtainable at the Lao border.
The bus/truck station is in the northwest part of the town.
Buses to/from Stung Treng take about 3 hours. As with all forms of transport in Cambodia, get your bus ticket early, and show up even earlier to ensure you get the best seat, as buses will fill up fast as soon as they let people board. With the improving road conditions, bus services have improved and there are now a number of options between Stung Treng and Phnom Penh.There are daily buses from Siem Reap to Kratie. They leave at 7AM and cost US$10. You will have to change buses half way through this journey at Skuon or Suong.
Share-taxis and minivans servicing Phnom Penh 4-5 hr, Stung Treng 2 hr, Kampong Cham 2 hr, Ban Lung 4 hr, Sen Monorom 4 hr, and a plethora of other small towns go from the taxi stand a block north of the Market. Prices to Phnom Penh and Stung Treng are about 20,000 riels per seat. These are much faster than the large commercially operated buses and comparably priced, though slightly less comfortable. Purchase an extra seat or share 3 between 2 of you for extra luxury. To and from Phnom Penh or Kampong Cham share-taxis and minivans can save several hours buy taking a road directly south of Kratie that most of the commercial bus companies do not use and bypassing Snuol.
Trucks go to various neighbouring towns and provinces but you have to weigh the benefits and drawbacks of this form of transport carefully. They are less safe and often slower than buses, as well as being more expensive. The one potentially overriding positive is that riding in a truck you get the chance to have a much closer connection with local Khmers than if you were riding in a tourist bus, particularly as you'd be surprised how many Khmers in the provincial capitals are somewhat proficient in English. Expect to wait a couple of hours for the truck to fill up, assuming you're the first one there.
to/from Kampong Cham: 10,000 riel in the back, 15,000 riel in the cabin, 8-10 hr
to/from Stung Treng: 20-25,000 riel in the back, 25-30,000 riel in the cabin, 10-12 hr
to/from Ratanakiri: US$12 in the back or in the cabin, 12 hr