You're getting some questionable advice here, so let's try to break it down. AA and UA work very differently.

AA:

You get one stopover per one-way award. The stopover must be at the North American gateway city - which is the last city in North America on your routing leaving the region, or the first city on the way back. To use SFO as a stopover, you'd have to fly directly onward to an Asian city (for example) without making any other connection (say, LAX).

You have to pick a date for the onward flight but you can change it, free of charge, to any date up to a year past the date you originally purchased the ticket, as long as the new date is at least 21 days after your original purchase date. (If you can't make the change online, sometimes you'll pay the phone booking fee, sometimes not.)

UA:

You get one stopover per round trip award. The stopover can generally be anywhere on a valid routing between the origin and destination.

Again, you have to pick a date for the onward flight and again you can change it free of charge, up to a year past purchase date, as long as that date is also at least 21 days past the date you make the change. (Same with the phone booking fee.)

For both, you also get at least one open jaw to work with, so depending on exactly your routing, you can do more clever things.

See also: the Mile Value blog, which specializes in clever routing ideas.

(Particularly http://milevalue.com/the-five-cardin...rlines-awards/ and http://milevalue.com/choosing-betwee...united-awards/ )