1. Drink only bottled waters, and eat only foods that appear sanitary. Obviously you want to avoid things like dairy as you do not know if it is pasteurized. You want to eat only fully cooked foods, and try to stay away from seafood. While many delicacies in foreign countries look tempting, an upset stomach or diarrhea could really ruin your trip, so eat smart. Eat or drink a food only if it has been boiled, cooked, bottled or peeled.
2. Wash your hands regularly and use anti-bacterial soaps and gels to clean your hands, especially if you are not sure if the water is safe. When you travel you may find yourself taking public transit more often than you usually do, or using public restrooms regularly. Protect yourself from the spread of disease and illness by caring along a small bottle of anti-bacterial gel.
3. Brush your teeth with bottled water. If the water is not safe to drink, it is not safe to brush your teeth with either, so make sure you take precautions when doing things that involve contact but not consumption of water.
4. Boost your immune system before you go. There are things you can do to travel in good health, for example, you can take a daily dose of a high quality acidophilus supplement, which acts as preventative treatment. Start buttressing your immune system during the countdown to your trip, and especially focus on the gastrointestinal system as that is the source of most travel related health issues. To do this, start on a probiotic, containing healthy bacteria, several weeks before you travel. Look for one that does not require refrigeration so you can continue taking it during the duration of your trip. Also make sure you are regular before you go by eating lots of fiber. Bring ginger tablets with you for upset stomachs, and make sure you have any medications you generally take for illness as you may not be able to find the same thing in a foreign country
5. Keep your energy high. Make sure you get lots of antioxidants, natural viral disease fighters that help protect your immune system from potentially damaging free radicals. Taking a supplement or energy booster like ginseng can be very helpful

Most large cities are made for walking. For example, when you visit London walking through Hyde Park and the markets of Notting Hill is a must. When you visit Paris, walking the market areas around Rue De Montague is a must. When in New York, walking the Brooklyn Bridge is awesome, etc. So, make sure that you are fit and healthy so that you can enjoy these walks, and your vacation without worrying about finding the nearest bathroom or getting in to see a doctor.