1. Do your research- Research the country you are going to visit. This helps in more ways than one. First the country will be more interesting if you know some of the history of the country. For example, I was in Ireland and our cab driver was pointing out one of the government buildings in which you can still see bullets from one of the English attacks. Because I knew the history, it was interesting to me. More importantly you should do your research as to be sure you DON'T OFFEND anyone! You may do something that is offenseful and have no idea!
2. Be aware of your surroundings- You most likely stand out as a tourist. Whether its your accent (or lack of one) or the way you are dressed (or the map you're holding, or pictures your taking). Be sure you are taking care of yourself. For example, don't leave your purse in a rental car...it will get stolen (with your passport and money).
3. Eating out etiquette- This is really important to understand before you go to another country. As a traveler you will most likely be eating out more often then not. Think about tipping for example. That is something that really changes from country to country.