Passports & visas
Once you've decided where and when you're going, even before you have an exact departure date, you'll need to make sure that your passports are up to date (with plenty of time before they expire) and to start thinking about visas.
Most countries require you to have at least six months' validity on your passport as a condition of entry. Renewing a passport while you're overseas is a pain, so if you only have about a year of validity left, speak to the passport issuing authority in your country of residence before you go and explain your plans. They will able to advise on the best approach for renewal.
If you know which countries you're visiting and when, arrange for as many visas as possible to be issued before you leave. Although this may be slightly more expensive, it will save a lot of hassle once you're travelling.
Although many countries are streamlining their entry requirements, either waiving the requirement for a visa or issuing them on arrival, pre-issued visas are still mandatory in many countries, and some also require you to have a valid ticket for onward travel.
A country-by-country breakdown of the visa requirements for British citizens is available on the UKgov website: https://www.gov.uk/foreign-travel-advice
If, like us, you have dual nationality, be careful about getting visas issued in different passports. We encountered difficulties when we tried to cross the overland border between Cambodia and Laos - the Laos authorities wouldn't accept our visa because it was in a different passport to the exit stamp from Cambodia.
Also be aware that some countries (including Malaysia and Indonesia) don't recognise dual nationality and will actually deport individuals found to be travelling with more than one passport.
Driving acoss overland borders in Europe is usually extremely straightforward. Many don't even require you to stop, while others may require you to show your passport and buy a vignette to use the motorways in the new country (for example, Romania, Bulgaria and Switzerland).
Crossing overland borders in the developing world, on the other hand, can often be more problematic. Crossing points tend to be hotspots for bureaucracy and corruption, with 'fixers' who offer to help streamline the process scamming travellers and officials often asking for bribes. For this reason, we always arranged visas ahead of time - either before we left or when we were visiting major cities.
When travelling overland between countries, you'll find that there tend to be more restrictions than if you travel by air. Many countries that offer visas on arrival at airports don't offer the same courtesy to those crossing land borders.
Getting visas while you travel
You may find that you need to get visas once you've embarked on your journey. There are anumber of reasons why this might be the case:
the visa can't be issued more than a few months in advance
you want to extend a visa that you've already been issued
you change your plans and decide to visit a country for which you don't already have a visa
You decide to get a visa before you arrive instead of getting a visa on arrival.
If you find yourself in this situation, make sure to plan ahead, as in many cases it will take a few days for your application to be processed. Check the opening times for the embassy and if time is tight, check for public holidays as embassies close for holidays in both the country in which they are located and the country they represent.
If possible, get your visas issued in smaller countries or cities as the demand will be smaller, meaning no queues, a shorter processing time, and often friendlier staff. For example, we got our visa for Burma in Phnom Penn rather than Bangkok as we had read that the process was much more straightforward in the former.
Take some US dollars and passport photos as well as your passports and any onward travel details.
We found that the embassies we visited were fairly relaxed about inbound and outbound travel details and didn't require any documentation. They were also relaxed about the details of the border crossings we planned to use and didn't hold us to them.
Take a set of passport photos for each family member with you. You can usually get them as you go, but it's very handy having them with you, and they don't weigh much. Carrying a bit of cash in US dollars also helps with the visa application process, as most places ask you to pay in dollars.
Before you leave, scan the identity page of your passport and any visas you've been issued. Print up a copy of each and carry it with you - in a different place to where you carry your passports. And keep electronic copies with you and in 'cloud' locations that you can easily access.