This is not intended to be a comprehensive guide, purely a starting point for consideration. We will display a more comprehensive guide when time permits us to write it.
TYRES: Believe it or not when planning to modify you 4x4, to improve its off-road capability, you start at the bottom and work up. To be more precise you start with the tyres. Your choice will dictate most of the other work required. Some choose the tyres purely on cosmetics, others for more practical reasons such as increased ground clearance and tread type. Often there may be some compromise depending on factors such as; clearance in the wheelarches and the knock on effect to the overall gearing. The choice of tyre will also affect your choice of rim..
SUSPENSION: Once the wheel and tyre combination is decided, the next stage will be the means to physically fit them under the vehicle. Usually a choice of larger tyre will go hand-in-hand with a suspension or body lift. If your vehicle has independent suspension this may be a problem as only minimal suspension adjustment (up to a couple of inches) maybe possible. However, assuming your 4x4 has beam type axles, there is probably a range of suspension kits to choose from. The Old Man Emu Suspension kit (includes springs and shocks), for example, will lift most things by approximately 2+". This may give you the required gain in height to fit your taller tyres. So now you have a lifted 4x4 giving that all important greater ground and underbody clearance.
When it comes to a choice of suspension package it is generally accepted that coils will give better articulation than leaf springs.
BODY LIFT: Great, except changing the suspension kit might not be the whole story. Usually with the raised suspension comes increased axle articulation which means that the tyres may foul the wheelarches at extremes of travel. Several kits offer longer bump stop rubbers that restrict the upwards travel of the suspension, however this reduces the ultimate axle articulation which defeats the point of improving the suspension in the first place. One answer to this is to raise the body with a 'body-lift' kit. This however, will only work if the vehicle has a separate chassis. Fine, if your 4x4 is a Jeep Wrangler or Land Rover defender et al, but not the monocoque constructed Cherokee or Freelander. So this may mean a re-think on the wheels and tyres front. Limiting the travel of the suspension slightly by increasing the bump stops may provide the cheapest solution.
DIFF-RATIOS: Assuming all is going well and you have a wheel, tyre and suspension combination that works (remember that increased vehicle weight through the inclusion of winches and heavier bumpers etc., may again reduce the now increased ride height, although some of the kits offer heavier springs to compensate for the extra weight). The next consideration is what affect the larger tyres have on the overall gearing of the vehicle. Larger tyres will have raised the gearing, which, is great if you want to be able to do 0-80 mph in first gear but not very practical, particularly off-road. Apart from changing the gearing from a practical point of view that could have you struggling uphill on and off road, you will also have reduced your engine braking which could be crucial to you getting you down a steep slope in one piece! To fix this you need to change the diff-ratios or transferbox ratios. Again this will depend on what is available for your vehicle. Also if you are considering fitting diff-locks to give you the edge in difficult terrain, now is the time to do it if you are changing diff ratios as most of the labour is the same.
So as you can see modifying your beloved 4x4 needs careful planning and although you don't need to carry it out all at once you need to have that long term plan. Also, as you can see, at any stage of planning it may be necessary to go back and re-think the previous choices.
For more details of the process, have a look at the article on the build up of Jeep UK's Jeep Wrangler TJ - Rubicon II.