Migrating to almost any country is a very difficult and restricted process. Many countries will allow easier access for populations in trouble, though -- either through war, political persecution, or, sometimes, poverty -- these change frequently.
If you're just a person who wants to move to the US, are from a stable country, and have no compelling reason to move here, you can still apply if you have someone to sponsor you (usually a relative), but you have to go on a waiting list and still might not be approved. At a minimum, that process takes about 5 years. Maybe we can eventually make language a complete impediment to understanding. -Hobbes
i actually knew an american once who lived and worked a whitecollar job in the UK and just went in and out of the country every couple of months as a tourist. for some reason she was able to pay taxes and everything without anyone ever finding out she's only here as a tourist.
she was quite paranoid though that she'd be found out and kicked from the country, which would result in a prolonged ban on entering I think. She even considered marrying a gay guy pro-forma who wanted easier access to the US in return.