Once you made a decision to be brave and break away from the perceived safety of a convenient tourist bus coming with pickup from your hotel or guesthouse, you will need to figure out how to get to the bus station and how to buy the right ticket.
The most important thing is that prices are fixed for all routes, so, no need to shop around the counters of different companies for a better deal, you merely need to find the right departure time and class for your needs. Cashiers and touts are usually quite good at pointing you at the right directions if you are confused, there is no need to be paranoid or feel cheated. If you are upcountry, it will be assumed you are heading for Bangkok. Counters usually only have signs in Thai, but on a large cardboard sign, they usually display their next departure time, so it is easy to make comparisons between fifteen competing companies. Cashiers speak enough English to sell you tickets for the class and time you ask for; however, if you are buying in advance, it is better to write down the required date and time just to make sure.
Ok, this is a tough one – bus schedule in Udon Thani

Bangkok is the most difficult. Absolutely no travel agencies would sell you tickets to government buses; they would even laugh at you and pressure you very hard to buy a ticket to a tourist bus. Then, new arrivals will be easy preys for scams like dropping people off in Surat Thani instead of Ko Samui or Krabi. Your best bet is just to turn up at the bus station. Buses leave at all hours to major long-distance destinations such as Phuket, Surat Thani and Chiang Mai. With other destinations, such as Krabi or Nong Khai, departures are early in the morning (6 to 8) and in the evening (6 to 9), with no or very few buses in between. For destinations that do not require a full day or full night or travel, such as Kanchanaburi or Trat, buses usually leave at regular intervals during the day (every 30, 60 or 90 minutes), but there are no overnight buses. Even if you check out the schedules online (e.g. at http://www.transport.co.th ) or call and ask ahead on the very same day, the actual departures are going to be different from what you were told. If there are lots of passengers, extra buses are added to the schedule. Unless it is a long weekend or a holidays, when seats are all sold out well in advance, you could simply go to the bus station and you will get a ticket for a bus leaving within two hours. It sounds slightly crazy to make the long way out to the bus station and merely hope you can get a ticket, but it has always worked for me in years of travelling thousands of kilometres.
In Bangkok, you need to know which of the three major bus stations to go to. The Northern bus station, or Mo Chit, serves Northern and Northeastern Thailand; it also has a number of buses to Trat and Pattaya on the Eastern seaboard. It is a HUGE bus station, with hundreds of counters and stands where buses leave from and arrive at all hours of the day and night. Unfortunately, it is not within walking distance of Mo Chit metro station, definitely not if you are carrying a lot of luggage or have no idea which way to head. You can get a taxi from the centre for about 120-140 baht, or take the metro to Mo Chit station and take a taxi from there. The taxi very conveniently drops you off right in front of the counters of bus companies going to Chiang Mai, the most important destination in the north. There is an air-conditioned waiting hall with restaurants, convenience stores, bookshop, left luggage service, shops. It is an overwhelming, confusing place to enter, with thousands of people, signs mostly in Thai: you might have the impulse to run away. However, there is an information counter where you can ask the counter number to your destination, and as there are very few foreigners in the crowd, cashiers easily spot them from a distance and show them which counter to go to.
Anything may happen at Mo Chit bus station! – Can you find out what class this bus is?

The Eastern or Ekamai bus station serves destinations on the Eastern seaboard, including Trat (for Ko Chang), Ban Phe (for Ko Samet), Rayong and Pattaya. Buses to Aranya Prathet, however, leave from the Northern bus station! This is a small, run-down bus station with only a handful of counters and stands. There are food shops and a 7-eleven, but no air-conditioned waiting area. The biggest advantage of this place is that it is right next to Ekamai skytrain station, almost in the centre of Bangkok (a 130-baht taxi ride from Khao San road though). Lots of tourists heading for the beaches in the east actually do make the effort to come here.
The Southern bus station, or Sai Tai, was relocated to a new spot in late 2007. Anyone who was familiar with the old location must agree that it was a nightmare, with potholes deep enough to swallow up double-deckers and traffic jams in the taxi queue even at 2 a.m. Its current location is on Borommaratchachonnani road, at the intersection with Phutthamonthon Sai1, far enough from central Bangkok not to be on any map. For Google Earth enthusiasts, here are the coordinates: 13°46’50.00″N, 100°25’35.01″E Now, this one is very difficult to get to, you definitely need a 200-baht taxi ride. This bus station serves the south, including Phuket, Surat Thani, Krabi, Hat Yai, Hua Hin, as well as the popular destinations of Damnoen Saduak and Kanchanaburi in the west. The huge bus station complex contains a food court (8 a.m. to 9 p.m.), KFC, McDonald’s, bag deposit service (5 a.m. to 9 p.m, 20 to 60 baht), internet cafe, bookshop, massage shop, and several shops selling all sorts of goods ranging from gold to fishing rods. It is easy to while away an hour or two waiting for your bus to leave.
In Chiang Mai, the bus station is a 30 or 40 baht taxi ride, or 10 minutes from the centre. Buses to Bangkok leave throughout the day, until 11 p.m. at night, with two peaks: there are literally dozens of buses leaving in the morning and in the evening, when cashiers will be competing to sell you a ticket (unless it is a holiday!) There are also buses to Chiang Rai, Pai, Sukhothai, Mae Hong Son and Lamphun at regular intervals. To Udon Thani, there are buses in the morning and in the evening only. I think in Chiang Mai, it is easy to opt for a government bus instead of a tourist bus. I have read and heard reports of tourist buses making the 9 to 10 hour journey in 14 hours because they included market detours! And I would not try to save 100 baht for the privilege of being stuffed into a 48-seater instead of the promised VIP bus.
In Surat Thani, all buses to Bangkok leave in the morning or in the evening. There is a peak between 7 p.m. and 8 p.m., with about a dozen sleepers heading for Bangkok. The only problem is that they arrive before 5 a.m., or, rather, maybe it is not such a big problem because you can catch a taxi to your hotel in Bangkok before the morning traffic jam. The bus station is outside town, accessible by local taxi (songthaew). The tourist bus station is in the city centre, of course, making very strong competition: I have never seen any other foreigner at this bus station.
In Krabi, the bus station is 4 kms from the centre, a fixed-price 20 baht songthaew ride. All the night buses to Bangkok leave between 4 p.m. and 5 p.m., the same time as tourist buses, with one big difference: you do not need to change buses in Surat Thani and wait around for hours. The only bus company running buses to Bangkok has an office in town, opposite the market. In addition, it is easy to get from Krabi to Phuket, Phang Nga, Surat Thani or Trang by second or third class buses, there are several departures a day.
Amazing view on the way from Krabi to Phang Nga – from the open door of the bus!

In Phuket, the bus station is in the centre of Phuket town. Local taxis mounted on large trucks instead of the small pickups get to the bus station from the beaches for 30 baht – instead of the crazy 400 baht or so quoted by the taxi drivers. As usually, most departures are in the morning and after sunset.
In some smaller towns such as Trat, Sukhothai and Ayutthaya, bus stations have been relocated outside town. There are always regular fixed price taxis to bus stations: ask at your guesthouse or hotel. When there are fewer tourists to hunt for, receptionists and agencies are more willing to tell you about government buses and sell you tickets as well. Small towns such as these are very easy to arrive at or get away from using government buses.
I think it is maybe too many details now š If you have questions, do ask at http://www.thailandqa.com/forum , somebody will surely be able to help you.
Generally speaking, I would advise you to go for it: when travelling around in Thailand, it usually looks more confusing than it turns out to be in the end. If the above photos do not absolutely scare you away, you can definitely do it.