I thought in Panama (and anywhere else outside the US) you have to pay income tax on earnings over $100k. I think you have to pay into Social Security too?
I'm Canadian though, so I don't have to deal with the long-armed-fingers-in-my-pockets embrace of Uncle Sam. Every other country in the world is sensible enough to only tax residents.
To be non-resident you have to spend at least half the year outside the country and have no residential ties to Canada like property, dependents, bank accounts, health insurance, etc.
See: http://www.cra-arc.gc.ca/tx/nnrsdnts/ndvdls/nnrs-eng.html
Not so the USA, which has a claim on all money earned by Americans world-wide. There's a ~U$100k earnings exemption (so many people aren't paying much tax back to the USA), but high-earning USA expats are in a different tax-world from their overseas colleagues.