In many ways travelling by bicycle offers a method of travel that falls between taking your own vehicle and using public transport. It is obviously slower than taking a motorised vehicle but can be taken on a lot of public transport. It also offers the flexibility to cover a greater area in a given time than walking.
The other advantage is that compared to taking your own vehicle you will gain more contact with local people and certainly in the third world will not be seen as a 'rich Westerner'.
The first thing is to decide what sort of terrain and the distance you plan to cover. For most road trips a light but sturdy touring bike with about 10 gears is a good option. Try not to go for anything to extreme not only might it receive unwelcome attention but exotic bikes will be more difficult to fix and obtain spares for. Steel frames have the advantage that they can be welded anywhere in the world.
Try and get the best set of wheels and tyres possible. Wheels should ideally use pre-stretched spokes. Make sure that the rims are a standard size and that tyres should be available at you destination. Expect to achieve around 8000 miles a tyre with a loaded bike.
Choosing a good comfortable saddle will definitely enhance your journey. Plastic saddles that do not allow moisture to wick aware should be avoided. Leather or suede saddles are good options but what ever type you use make sure to break it in before setting of on your trip
Although mountain bikes are very much the in fashion for touring the upright handle bars are not as good as drop handle bars which allow you to change your riding position when your muscles start to ache. Upright handle bars also cause more weight to be transferred through the saddle.
Rather than buying and transporting bicycles you could always hire them at your destination. The most important thing is to give the bike a good examination and if in doubt do not take it. Remember to ask for a lock for the bike. In a lot of countries bikes are cheap to hire but don't expect to get a flash new bike. The benefits are that you can explore a greater distance on than on foot but you obviously don't have the hassle of transporting the bike. IF you are using the bike in town and plan to have it for a couple of days it is worth checking where you are staying if there is some place you can keep it.
The golden rule as always is to keep your gear to a minimum. For most trips a change of clothes, tool kit, spares and a sleeping bag should suffice. The more weight you carry will not only make pedalling harder but will also cause more stress to the bike.
It is best to take layers of clothing using the same principles as for walking. Clothing should wick away moisture from the skin so fabrics such as Polartec is ideal. Your outer layer should be wind proof, breathable and waterproof here Gortex is a good option. Close fitting clothes will cause less drag and be more comfortable. Specialist cycling shoes are often very uncomfortable out of the pedals so a shoe with a solid sole may well be a more flexible option
The amount of spares you take will depend on the trip but in most countries you will be able to get replacement parts. Therefore a spare tyre, spokes, inner tube and puncture repair kit are the basic spares.For longer trips a spare chain might also be included.
Being able to repair the bike is important so carry tools to allow you to undertake any necessary repairs. A basic tool kit should consist of allen keys, chain link remover, screwdrivers, spanners (adjustable or multi size).
You will find that you need to drink plenty of liquids constantly especially in hot climates. Make sure you can obtain and if necessary purify enough water on for your trip before setting out on each stage.