The motor bike has both advantages and disadvantages over other vehicles. The main disadvantages are the lack of carrying capacity and being open to the elements at all times. The advantages are cheapper investment and running costs, ability to go anywhere and cheaper freight costs.
The choice of motor bike will depend on many factors. The main factor to consider is the type of road surface you are going to be riding on. For the developed world where you will be mainly on tarmac surface a standard road bike will be fine and give you a comfortable ride. If you are of to the developing countries where dirt tracks are the most common surface then a dirt bike might be a better option.
Engine capacity is the next consideration in general a 500cc engine will probably be sufficent but if you are riding with a passenger then 750cc will be a better option. Avoid single cylinder bikes as there is a greater chance of total failure. BMW's are the preferred bike for most overlanders but Japanese bikes are also worth considering. Try and find out the availability of parts for different makes in the countries you will be travelling through as this may have bearing on your choice.
Check the amount of weight the bike is designed to carry and never load it to that limit. For rough surfaces the limit should be derated to 70% to help prevent damage to your machine.
The chance of having a serious injury from an accident is the biggest reason not to ride a motorbike. Self preservation should be any riders main concern keep speeds down especially off tarmac. It may be great fun to speed down dirt tracks at home but a accident in the desert or rain forest hundreds of miles from a hospital can very easily be fatal even when you are travelling in company. Even when driving sensibly remember that local driving standards might be at best poor. Animals can be a serious threat especially if startled. Hitting a families cow might deprive them of food so don't expect any sympathy.
It goes without saying that a helmet and protective clothes should be worn however hot the conditions. In hot climates care needs to be taken to prevent dehydration.
Weather conditions especially at altitude can be severe, cold will be increased with wind chill as you travel, make sure you have enough clothing to allow survival should you break down.
Wearing protective clothing will protect you but can also alienate you from the locals to whom you may resemble an alien clad in leathers and helmet. The only real solution is to wear you leathers between towns but to remove them when entering towns especially if you plan to stop. If you stop to speak to anyone then remove your helmet before you approach them, if you have a passenger then they should remove their helmet even if not included in any conversation.
In parts of some countries motorbikes may never have been sen before so they my cause fear and will defiantly cause curiosity be prepared and keep calm do not shout at anyone this will cause a situation. Stay calm and be prepared to move on rather than get into a confrontation with local people.
Having a dirty basic bike will cause less interest than having a flash brightly coloured machine.
The preferred method of storage is to use metal boxes securely fastened to the bike for most items but having a small rucksack will enable you to quickly remove your valuables when leaving the bike. Try and pack heavy objects as low as possible to help keep the centre of gravity low and balance the weights on either side to help handling. Fragile items such as camera as may need protection from vibration and dust.
With all travel packing lightly is important with a motor bike this obviously has benefits from the point of view of handling and fuel consumption. The less personal gear you carry means that more spares and supplies can be carried.
Being able to repair the bike is important so carry tools to allow you to undertake any necessary repairs.Carry as many spares as possible spare chain and link removal tools are essential. Chains should be cleaned regularly especially in dusty and muddy conditions.
Good quality air filters should be used and fuel filters fitted. Low compression pistons allow poorer quality fuels to be used.
A good puncture repair kit will keep you on the road and if you have more than a single cylinder a Schrader pump which uses one cylinder to power it by removal of a spark plug is a good investment. A set of tyres should last 19,000 miles depending on surface, riding style etc.
In most countries you will need the same paper work for a motor bike as you do for a car this includes carnets, international driving licences etc. In some countries you may not need insurance but it makes sense to be covered even so. It can often be obtained at borders but try and arrange it before hand if possible.