The amount of preparation you undertake on a vehicle before you leave on your trip is obviously dependant on the type of trip your going on, age of vehicle and expected availability of parts. This page looks at the preparation for long over land trips and a lot of what is mentioned may not be applicable to your trip.
When under taking any initial preparation only use the tools and equipment you will have with you on the road. This will help you learn to adapt to using just the tools you have and also bring out any short falls in your equipment.
If your interested as to what spares and items you should have in your tool kit then check out the expedition packing list.
It makes sense to buy your vehicle several months in advance so you get used to driving it and also have a better understanding of any potential problems before you start your trip preparation.
If the vehicle is fairly high milage then the initial thing to consider is an engine rebuild even if you feel a total rebuild is out of the question taking the head of and having a quick assessment of the internals of you engine should be a minimum. A part from anything else its much easier to figure out how it comes apart in your garage than in the middle of the desert. For the gear box the rules are pretty much the same. Remember that on a four wheel drive changing the clutch can be hard work even in a well designed garage so give it a close inspection and if there is any doubt that it won't last the trip then change it.
If your going to be travelling in dusty conditions then a snorkel is worth considering for your air intake these lift the height of the air intake to roof level so that the amount of dust that finds its way into the engines air filter is reduced. They also help prevent water getting into the engine when crossing rivers etc. For crossing rivers or wading of any type in your vehicle, plugs should be taken for all the breather holes in the transmission system or alternatively a wading kit fitted, this is a selection of pipes that lift the breather holes out of the way of the water.
If the vehicle is to be heavily laden then up grading the suspension and shock absorbers should be considered. Replacing all suspension bushes should also be considered especially if on close inspection they appear at all worn. The main problem with this is that extra stresses can be applied to the chassis so this is not recommended. Check the manufactures recommended maximum load weights.
Fit a second in line fuel filter and carry some spares the quality of diesel abroad is often of lower grade and can be contaminated due to poor storage conditions.
Long range fuel tanks can be fitted or jerry cans can be used to extend your range. There are pros and cons for each method. The best is probably a mixture of the two. If a second fuel tank is added it is advisable to have a stop tap on it so that it can be isolated from the main tank. When travelling off asphalt roads the tap should be closed and only opened to refuel the main tank. This means that if the main tank becomes holed all the fuel will no be lost before you realise what has happened. When using jerry cans on the roof rack use a suitable length of hose to siphon the fuel into the main tank this saves the effort of getting them on and off the roof. Often it may be possible to refuel them with out taking them off the roof. When carrying a lot of fuel on the roof try and use the fuel alternatively from each side of the vehicle to help balance the weight this is also the case with water usage. Remember that fuel and water are probably the heaviest items carried so care should be taken to balance the load evenly on the vehicle. If the vehicle becomes stuck then removing them will also lighten the vehicle adding recovery.
Adding a second split charge battery is a good idea. This second battery should be charged via a relay system so that when the engine is off it is disconnected from the main battery. The advantages of this is that the secondary battery can then be used for powering lighting etc while parked up without the worry of draining the main battery so the vehicle won't start in the morning. If the main battery fails for any reason you also have a back up battery.
Low voltage fluorescent lighting should be used at night as fluorescent lamps are far more efficient than normal bulbs. External lights if not permanently fitted could be connected to a external socket this is easier and safer than trailing leads through the windows and doors of a vehicle.
Additional driving lights can be added but if the conditions warrant using them then the chances are its safer not driving until the conditions improve
If the trip is to include off road or dirt road travel then consider under body protection for your vehicle. Main areas to protect are the steering, differentials and fuel tank. Although no protection is an alternative to defensive driving. If you're likely to be clambering over the front wings to reach the roof rack then you should also consider protecting this. Most people will fit bull bars but you should consider if they are really necessary, just use mesh light protectors they weigh a lot less! It is inadvisable to use rear door mounted spare wheel carriers since they tend not to with stand the stress of off road usage. You can get systems that take the strain off the rear door hinges alternatively use bonnet or roof mounted carriers. Foliage clearance cables for off road use may be needed these extend from the bullbars to the roof rack and help deflect branches from the wind screen.
With all your possessions in a metal box on wheels security should be of prime consideration. Never leave a door unlocked wether you are inside or outside of the vehicle even if you seem in a good area. If you are in a town known for crime then always leave some one in the vehicle and try never to let it out of your site. Fit safes for you documentation and money. These don't necessarily have to be strong just well disguised a good idea is to put some of your funds that you won't be requiring for a few months in a metal box of the type sold by electronics retailers (Maplin, Radio Spares) and bolt it in the engine compartment, painting it to camouflage it with the rest of the engine. This is also a good way to hide money you are not declaring at borders since officials don't like to get their hands greasy. Internally old machine gun round boxes available from army surplice shops make good safes bolt them under a seat and drill a hole for a padlock. Remember in a lot of countries banditry is increasing. If you are stopped at a road block by non officials then put that bull bar into usage and drive though it.
Fit a basic alarm system consisting of wiring the horn or a siren to the courtesy light switches (you may have to add another switch for the back door), with a key switch mounted on the exterior of the vehicle. Its not advisable to go for a complicated alarm system as you can't fix it when it goes wrong. Similarly a simple engine immobiliser can be made by taking one of the wires from the ignition or fuel pump and placing a hidden in line switch when you leave the vehicle just remember to flick the switch. You can install both these systems for less than the cost of a couple of rounds of beer.
Adding extra locks is also worth the effort the simplest is to add pad locks to all your doors and remember to add one or two to the bonnet (hood). You should find that you can get a set of padlocks which take the same key from a locksmith. Anything that's not locked down might disappear so remember items on the roof rack, use steel cable locks on them.
Rear and side windows should be capable of being firmly secured and adding external mesh protection will help prevent thefts and also deflect branches or stones thrown by local kids. You could also consider adding an internal secured cargo area of mesh.
When the inevitably happens and something breaks and you haven't got and can't get the part you want where you are then the only option is to get it sent out from home. Anyone who has ever bought parts for the vehicle will know that always seems to be lots of options for the same or similar parts. It is therefore important to gain as much information about the exact engine, gear box etc that you have. This information should be left with someone at home ideally a mechanic or at least someone with good knowledge of vehicles. Even better if they've helped you prepare the vehicle in the first place. It is also worth leaving them a copy of a service manual and taking the identical manual on the road with you so that you both know you are talking about the same thing. In the UK the Hayes manuals are an excellent option for this purpose. If you're not in the UK and can't get hold of Hayes manuals then try amazon or one of the other on line book shops. It's also worth faxing the information rather than communicating by phone as its much more precise and there is less chance of errors being made. Finally get the part sent out by a reputable carrier Federal Express or DHL.