On the road:
1) Drink water
2) Buy water as soon as possible when landed.
3) Drink water
4) Make sure there is enough drinking water the night before for the next day.
5) Drink water
Reminding myself to drink water constantly has helped prevent getting ill on the road. I have a Platypus water bottle tucked into each travel bag and one in my tote ready to be filled from a larger bottle I buy at a convenience store (or from the plane's refreshment trolley). You can never have too many Platys (though I dislike Vapur)
I find that once I get dehydrated on the road, it's almost guaranteed that I will fall ill.
^^
On a trip a few weeks ago with questionable facilities everyone else was trying to avoid drinking in order to not have to use said places that frequently. I however was constantly filling up with water - but it kept me from having any issues (and the facilities weren't THAT bad - most days). Lesson was learned from a similar trip last year where I suffered problems as a result of not drinking enough water.