You buy a passport or you go to a country where you can get permanent residency and they you stay long enough to qualify for citizenship. Or yeah. Marriage. :-)

What is your opinion of the "economic citizenship" programs in countries such as Nevis & St. Kitts, Dominica, etc?

Those are the traditional choices.

Switzerland's cantons will let you work out a deal on citizenship and a flat rate on how much taxes you pay.

I know that for Australia and Canada, at least, marriage really means nothing as far as permanent residency (and from that, citizenship) is concerned.

It's all about the relationship itself. That is to say, you need to prove that the relationship is solid and marriage does not prove that. It's the years before that of living together, joint bank accounts and bills that proves that relationship.

* solidly American (e.g. 3rd+ gen),*

and where exactly does one's US citizenship start becoming "unsolid"?

He doesn't mean it like that. He's referencing the tendency of other nations to grant citizenship to descendents of citizens. E.g., if my father were German, I'd have a better chance of acquiring German citizenship than if my most recently immigrated ancestors came to the U.S. circa 1840 (which for me happens to be the case).

I read it more to mean, a US citizen with highly attenuated ties to another country; after 3rd generation, you aren't likely to have immediate family overseas, &c. At least, I hope that's what was meant.

Yes, this is what I meant.