Travel Tips : Going from A to B within Thailand : Taking the bus
And believe me there are busses going from/to almost every place in Thailand. It is really amazing. Some weird long haul connections that can be made from all the way in South Thailand to the border with Myanmar in North Thailand.
Personally I am not a bus lover. Yes it is cheap, but flights can be cheap also. So I would not take a bus from Bangkok to Chiang Mai but the train or airplane. But going to Bueng Kan in North-East Thailand you will have no other choice as at least a part by bus.
While traveling by bus you should consider safety and comfort. In my personal opinion ... long haul trips with the bus lacks both of them. Especially when taking the bus at night. It is much safer to travel in day-time.
My favourite bus company is "Nakhon Chai Air" because they drive safe and have a very low accident count. Due to the air-condition in the bus I would recommend to take some warm cloths with you. And I would recommend to keep all your real valuables near to you (on your body). Personally I would never take a bus like on the photo, this are most time charted busses for schools or companies. They drive like crazy and most times have very loud music inside.