My niece and her husband are going on a Rhine River tour this summer. He’s a real redneck and looks with suspicion upon wine-slurping, microbrew drinking pinkos. I told him that he’d have to spend the entire trip drinking German beers, and that they use MGD over there to clean their paintbrushes. He’s none to happy about it.
As a counterpoint to-- but not necessarily a disagreement with-- the “make sure you go everywhere and go to all the most local places and talk to every single person” strain of thought, I would say also make sure to know your place. Sometimes, just because people are being friendly with you doesn’t mean they actually want you there. Travellers are simply not entitled to see every single place, event, or person. Just because that one guy was really nice and showed you around the temple doesn’t mean it’s actually ok for you to be there. Even though it’s nice to think of the entire earth as the patrimony of all people, it can feel like a colonialistic idea; every community has places that they’d rather interlopers not make a scene of.
Never eat anything you can’t pronounce. 
Try to find a phrasebook, if only for the medical emergency section. Trying to pantomime ‘I need something for a yeast infection’ at a pharmacy rarely works. Pantomiming anything at the emergency room never does
Buy a "Rough Guide"of the country(s) you are visiting as it will give you a more honest account of what to know good or bad then those guides of the “jolly the tourist” variety.
"Lonely Planets"are the next best IME.
Have photo copies made of your passport,tickets,insurance and other vital travel documents and KEEP THEM SEPARATE FROM THE DOCUMENTS THEMSELVES in case you get robbed or lose your luggage etc.
When travelling in the third world some people carry their own sterile needles in case they need medical treatment ie.transfusions,injections etc.
It is not uncommon in various countries for medical products to be “recycled” without the bother of sterilisation.
Learn how to say hello,goodbye,please, thankyou etc. and HOW TO PRONOUNCE THEM PROPERLY,it does go a long way but only if the locals realise that you are speaking in their language and not mumbling what is to them gibberish.
Learn the local “NoNos” as in not pointing your feet at a Buddha or touching people with your right hand whatever.
Never look helpless,confused or gullible because if you do you will soon attract the attention of the local sharks.
You may be white and middle class but outside of tourist areas you may experience for the first time in your life genuine hard core racism from the locals.
I have also seen an African American couple in E.Africa subjected to racist treatment by the "African"Africans which totally bewildered them.
I dont mean my few pieces of advice to worry you,I have travelled extensively throughout the world and loved it,had some totally amazing experiences and met some incredible people, but whenever I read advice on boards like this along the lines of “all people are basically saints who just happen to speak a different language” or just throw yourselves on peoples mercy and they will look after you" I always suspect those who give that advice either haven’t actually travelled very much(if at all)or haven’t really have had a clue what has been going on all around them at any given time.
Good luck on your travels,I suspect that it will be one of the greatest experiences of your life.
I am just so glad that I went off travelling at a young age to aquire what has become a lifelong habit that I will never grow out of.