Make sure you are well rested, eating properly, and exercising, so you'll be in top form for your trip. It is also important to note that it is quite common to suffer from mild diarrhea and even vomiting upon arrival. This is due to the shock because of the change in environment. Make sure to have a few extra days in your itinerary so you can get acclimated, especially in high altitude areas like the Andes. Also keep in mind that in some Latin American countries the tap water may be contaminated. Try to drink and brush your teeth with purified bottled water and avoid drinks that contain ice.
First-aid kits are widely available at pharmacies and health clinics or you can put one together yourself. Make sure to include antiseptic cream, antihistamines for bites or allergies, antiseptic soap, insect repellant with DEET, painkillers (ex. Ibuprofen), band-aids of different sizes, rehydration salts for dehydration, sun block, and tweezers for splinters. These items are also available at pharmacies in all South American countries.
If you are not a frequent traveler to South America you will need certain vaccinations before you go. Try to get your shots at least three months before you depart, since some vaccinations require more than one dose. It is also common to feel slightly under the weather after your shots, but this doesn't last long. You will need to be vaccinated against typhoid, hepatitis, yellow fever, and rabies. Consult your public health department or local doctor for more details.
It is important to be informed of the news in the country of your visit. Also make sure to have the contact information of your nearest embassy or consulate in case of an emergency.
Keep your passport with you at all times. In fact, in most South American countries it is illegal not to carry some form of identification. Before your trip make photocopies of your passport. This way it will be easier to replace if it gets lost or stolen.
This is probably the most common form of crime against tourists. Make sure to keep some emergency cash separate from your wallet just in case it gets stolen. Be extra careful of pickpockets at crowded places like airport terminals, fiestas, markets, and city centres.