Abiding by foreign customs while travelling is both the least you can do and the hardest thing to achieve. Whether it's showing the soles of your feet in Thailand, leaning up against a Maori table in New Zealand or eating with your left hand in India, the pitfalls and pratfalls are too numerous to mention.
The basic rules are obvious: do read that boring bit at the beginning of the guide book about customs and etiquette in your chosen destination; err on the side of caution when it comes to revealing clothing, and keep your eyes peeled for how those around you are behaving.
If the monks around you are silent when you visit a Buddhist temple, follow their lead; or, at the other end of the politeness spectrum, if you're trying to get bus tickets at a bus station, don't waste time queuing politely in a line if the locals are scrummaging in a free-for-all. Adapt the usual maxim and do as you can see others are being done by.
Try to resist the closed attitude of thinking that your way is the right way, and above all, avoid the arrogance of imagining that everyone on your travels speaks English - or should do. A good starting-point is to have some humility about your own inability to speak anything in their language beyond "How much is this?".
Always bear in mind Mark Twain's observation, 'I have found that there ain't no surer way to find out whether you like people or hate them than to travel with them'. Travelling with even your greatest friend or lover can place unexpected strains on the relationship, largely avoided if you adhere even more scrupulously than usual to the rules of living together.
Little things like keeping your living space tidy and clean, sticking to joint plans, making sure you're as punctual as possible, adhering to an agreed budget - these tiny observances can make or break your travelling experience.