We had a bad experience with s group of foreign 'tourists' sitting on the boat's floor in the front who didn't let other people to the same public boat in LPB direction. They had obviously decided there is no more place for others and by no way they would let anyone board by loudly 'singing/screaming' NO and GO AWAY to people who even tried to get too close. It is usually boat owner/captain who decide that his boat is full enough and ready to leave. We did ignored their behaviour and sat down despite their screams and angry faces as we wanted to go to Hongsa direction, about an hour down the Mekong. The irony was there was still enough of place (seats) for at least 15 people or more withour getting overcrowded!!! All those poor backpackers who were chased away (and tried to stop us as well, because they didn't try by themself) could board in and it wouldn't harm anyone plus the owner would not loose money. I was told laters they read in Lonely Planet to ask for more boats if there are a lot of people. However, that behaviour of 'rich' foreign tourists is unacceptable and impacts Lao society very negatively.
Unique Suggestions: Always ask the captain of the boat if there is a place for you on the boat. And warn tourists that loud and rude behavior is not welcome in any part of Laos.
Fun Alternatives: If boat is too full, the captain will leave anyway and won't let more people to board. As the latest public boat leaves 9.30, you will have to wait for another day if you missed that one. If your route and time is flexible you can take public bus to Oudomxay and enjoy wonderful landscape between the two cities.
There are plenty of children near the port when there are boats landing for overnight stay. I've seen some people will use children to carry huge backpacks or bags upstairs and giving them 1 or 2 baht for the 'service'. I don't think that's really fair paid, but many travelers or tourists who just came from Thailand believe they have to pay less than there or perhaps they believe things and small services are for free in Laos. I haven't seen those people would complain when they pay 20 000 KIP for a beer in a bar.
Unique Suggestions: If you have to take children's sevice for carrying your bag, pay at least 2000-3000 kip or 10 baht.
Fun Alternatives: Pay fair or don't take their service.