Any tips for travelling in South America?

Knowing that you don't know can be alarmingly scary. So we've got a lot of questions and quite a few assumptions:

• Safety - be careful and sensible, yes. But are there any spots in BA, Santiago, Cusco, Lima, Iquitos etc we should best avoid? Any suggestions for a really practical money-belt? (Even considering strapping valuables underneath my ample breasts - but that's just plain silly.)
• Lolly - we're thinking a combination of cash (dollars?), credit card, travellers' cheques and debit card to raid the holes in the wall (yep, got the Nationwide account that doesn't charge for overseas usage). Update: Following George W's renewal of the trade embargo, the dollar carries a 10% "charge" in Cuba so sterling and euro are the way to go, apparently. And the ATMs are not the way to go - telephone lines are down most of the time.
• Food - whereas I fully expect to loose at least a stone over the four months it would be good to remember the trip for its people, places and activities and not for a lingering whiff of vomit.
• Inca Trail - want to use a local trail operator, and not an exorbitant UK-based one. Any suggestions for a responsible, environmentally aware tour company in Cusco that treats its porters well?