The easiest and most convenient way to get from Northern Thailand to Laos is to take the Slow Boat down the Mekong River, into Luang Prabang. It’s only $9 and takes three days, but offers beautiful scenery – highly recommended: unspoiled traditional villages, fishing communities, paddy fields, etc.
However, right when you get into the port to take the boat, you will be approached by bus drivers trying to do absolutely anything to get you off your mindset of taking the boat: “Last Friday somebody caught malaria in Pak Beng (the stop-off village) and died on Sunday”, or “You will be robbed there”, etc.
If you take the bus instead of the boat, there is a big chance that you will be either squished with a bunch of locals (the bus is very small and packed), not even have a seat, and forced to smell all kinds of odors.
How to avoid: if you really want to take the bus, use the most reputable companies, not the ones that are pulling your sleeves. The slow boat is a better choice, anyway.