By ship
Fast boats used to be the best way to get here, but Chinese money has made the roads a joy at least compared to their old state!.Regular boat services north and south no longer run, though small speedboats can be chartered for trips to non-standard destinations.
By road
National Highway 7 from the Laos-Cambodian border to Kratie via Stung Treng is fully sealed now. So it`s not a problem any more to do the trip by bus/ minbus in either direction. Every guesthouse will sell you a ticket. Keep in mind that it can be quiet difficult to get public transport directly at the border, so it`s worth considering buying a ticket to your "final" destination. The minibus to the border takes about 1½h, while it`s a 4-5h drive to Kratie much more in the rainy season. Minibus drivers have been known to abandon their passengers at the border crossing at Dom Kralor.
The daily bus from Phnom Penh to Laos passes the town in the afternoon and is the safest option to get to Laos.
Trucking from Kratie is also an option, and will set you back 20-25,000 riel for riding in the back of the truck, and 25-30,000 riel if you wish to ride in the cabin. As discussed on the Kratie page, trucking is somewhat less safe than other modes of transport, and can take much longer. However, trucking in Cambodia puts you in much greater direct contact with the locals as it's locals you'll be sharing a truck with most of the time, and isn't that one of the purposes of travelling?