[Avoid jetlag and airsickness] Carry the correct forms of currency
Exchange just enough money to cover your expenses for your travels. There's nothing worse than being stuck in a new country on your first day with no local currency. Make sure your ATM card can be used in the places you are visiting (you'll find a logo like the 'Cirrus' one on the back of your card) - some places in South America won't accept this so ensure you have enough cash on hand. It is safest to take a mixture of currency with you - a credit card, some cash, and perhaps some travellers cheques. This will cover your butt just in case that credit card declines or a machine won't accept your ATM card.