Travelling by buses is usually one of the cheapest methods of getting around a country. In most countries you can get a bus to all major destinations and most minor destinations. Buses come in all shapes, condition, level of comfort and likely hood of holding together long enough to get you to your destination.
Buses generally come in two forms what the Westerner considers a bus and also mini buses. The smaller mini buses will often do the shorter routes while the buses will do the longer journeys.
When planning your trip try and take into account local holidays and for example in the Muslim world it can be a nightmare to get on any form of transport at the end of Ramadan as people tend to visit their relatives.
With most buses its possible to book ahead it is best to do this when you arrive in a place or at least check out the availability. Routes are often booked up and if you leave it to the day of departure you may have to wait several days for the next vacant seat. If you find the bus to your next destination is full try looking for an intermediate town where you may be able to get a connection to and out of.
In some countries you will find that different companies run buses along the same route and it may be worth shopping around the different firms to get the best fair. Depending on the part of the world it might also be possible to book smoking or non smoking seats. Due to the nature of driving in a lot of the world seats at the front and rear can be a dangerous place if an accident happens not that the middle is much better!
If you arrive at a bus station but can't find bus to your destination check that there isn't another bus station serving a different part of the town.
Mini buses (bush taxis) in general tend to be either 12 seater mini vans or large Peugeot cars. These are usually more expensive than a standard bus and are owned by the driver so tend to often be in a poorer state of repair than a state or company run bus. However they are usually quicker and more flexible. Generally they leave when full and don't run to a time table. They will run from a bus depot in major towns but in smaller towns you may have to walk out to the main road and flag one down. Even in larger towns they will often drive around the town picking up customers before heading out. In general you pay at your destination or just before it. Some will have 'conductors' or on some you will pay the driver. Most routes have fixed fares and the drivers won't rip you off however check the price with another passenger. In Egypt and probably other tourist areas the drivers try and get you to pay more but its very easy to get other passengers to back you and the driver usually backs down. If he won't place the correct fare into his hand and walk away (Check you have suitable change before you travel).
The main problem is that when busy a driver who 1 second ago was asleep will suddenly wake up yell out his destination and the mini bus will be full and gone before you have chance move. In this case it is often worth finding a local heading to the same place to help you. Expect to push, shove and squeeze to get on board if you give and inch you'll be waiting for the next one.
Having got on the bus assuming its not double booked you might find hostess service in western countries or in the third world a goat placed on your lap while its owner gets her children on board. In the third world the bus may not leave on time, stops might be made for no apparent reason for example so the driver can visit a relative. There may be no heating so if travelling in mountainous areas make sure you have good clothing. Although stops will be made for calls of nature (women to the left of the bus men to the right!) food and drink might be more of a problem so try and take some with you. In the third world a bus is often powered by the 'Will of Allah' so don't expect to get there on time and be philosophical when it breaks down.
If your destination is not the final destination of the bus make sure the driver knows where to drop you off. If possible also get another passenger to tell you when you are near your destination.
Finally having a sense of humour is essential after all that goat might not have the same bowel control as the rest of your fellow passengers. Remember also to be polite to other passengers even when they get on your nerves and to respect local traditions. For example in some countries it is considered impolite to sit next to a woman who you don't know.
Wherever you are travelling it is worth splitting up your luggage in to essentials (camera, documents etc.) and the rest. The essentials should be kept with you at all times where as the rest of the gear can be stowed for the journey.
In the western world luggage is not to much of a problem. The main rules are to keep it in safe have it labelled and check it goes on to the bus, tell the driver your destination as the luggage may be placed in a different locker for each destination.
In the rest of the world things are generally not as simple the first thing is to keep an eye on your stowed luggage at all times especially when luggage is being loaded and off loaded on route. Never be tempted to leave luggage inside a bus even if the driver assures you that it will be locked up. In some cases your luggage will be travelling on the roof of a bus so make sure that essential gear and clothes are in water proof bags and remember bags might get dusty and dirty. Getting an old grain bag or thick bin linear to wrap your stuff in will help and may make it less appealing to a thief.
When crossing boarders it is down to you to have all the correct paper work. It is also doubly important to make sure no one has tampered with your luggage. Fares for buses crossing boarders tend to be high and it is often cheaper to get a bus to a boarder walk across and then pick up another on the other side.