Heya, these are some of my travelling tips. I'm not claiming to be an expert, but these are some things that have served me well when travelling in China, Hong Kong, Taiwan, Indonesia and Bangladesh and I hope that these may be of help to you too!
Your hands are dirty. Let me say that again; your hands are dirty. Try to keep them clean, yes, but always assume they're dirty. As a rule of thumb in this part of Asia, people's left hand is dirtier than the right, but after Western standards, they're both dirty.
This means, never eat with your hands. If eating a cookie, use a napkin around it, use the plastic wrapping etc. When eating dumplings from your favourite baozi stand on the corner, use the plastic bag the dumplings come in to hold the bun you're eating.
In the toilet, use some paper to lift the seat, or use your foot to kick it open (gently does it).
Always have water at hand (or in your hand ;-). I like to always have some in reserve in case I can't get another bottle at the very second I want/need it. Having a big bottle (Chinese: da ping shui) or two normal ones (Chinese: lian ping shui) makes sense.
Food's defintely on my top three list when travelling in Asia. In Taiwan & China (and to the degree you can find street stalls in Hong Kong), eating from stalls in the street and in 3m2 big restaurants are the best places to do so. Always seek out places that are busy, where locals go. If there's tons of people, it means that the food is made fresh. If it's made fresh, it's safe to eat as all bacteria die at 70C. Which leads me to my next rule of thumb, always eat hot food.
Probably the most dangerous places to eat are slow tourist restaurants. First of all, the food is not made fresh, but just as important; there's few/no locals eating there; the restaurant doesn't need satisfied customers to stay alive as all the visitors are one/two time guests anyway.
On the contrary, the old woman in the street that makes banana pancakes from her wee stall has done this 10 hours a day for the last 20 years. She makes this one dish and makes it well. That's why locals buy from her every morning at six on the way to work. And this is why you should feel (pretty) safe eating her food
In Bangladesh, I found eating from street stalls something to be avoided. Indeed, even Bangladeshi friends avoid them at times. It's better to seek out good restaurants where the hygiene is taken (more) seriously.
Most people get diarrhoea during the first two - three weeks when travelling in countries with a very different bacteria flora than what they're used to.
When/if you get diarrhoea, don't panic. Rememeber, this is totally normal and something you'll get through within a day or two. Just put your plans on a hold for a bit, look through all the photos you've taken so far on the trip, relax and follow the tips below.
Albeit I've got a good stomach for travelling and seldomly get diahorrea, I always prepare for it :-) These are some artifiacts I carry in my rucksack: