The most common ways are:
Immediate family who are USC's (such as parents)
Work visa - company sponsorship, requires a degree or a minimum of 12yrs progressive experience in a specialised area.
Investment of a minimum of $500K which is likely to be lost.
Buying a business (although this doesn't lead to a green card)
Transferring offices e.g. UK office to US office.
Those are the basics...obviously it's not quite as simple as that ;) We are here as a company sponsored my husband for a work visa, his job is very specialised and hardly any people in the world are qualified to his level, he doesn't have a degree but did have 15yrs of progressive work experience and over 40 specialised qualifications ;) Thankfully the company has started the green card application as that will take around 6-7yrs!!!! madness!!!
Bookmarks