To Pak Ou caves by boat
At the end of Pak Ou village, on the Mekong shore, it's easy to negotiate a ride by boat across the Mekong. The boats are very small and unsteady, but for the 3 minutes it takes to go across it's feasible: just cross your fingers and don't balance yourself. Local people normally ask for 5000 kip each way, per boat.
To Pak Ou by jumbo
While most people get to Pak Ou by boat from Luang Prabang, I decided to go by jumbo - which is basically a tuk-tuk - and stop to see two minority villages on my way there. The road is not paved, so it's only recommended if it's not raining - if it does it turns into a muddy slippery track which could become dangerous in some spots, since there's no fence between you and the waters of the Mekong down below. This trip is very scenic: it takes about 50 minutes to get there and costs about 10$. You'll find jumbo drivers along the main street in Luang prabang, and it's really easy to negotiate a ride.