Starwood Preferred Guest’s quarterly promotion (“More for You”) has become active. You have to register before November 30th, and you will earn extra points for stays between September 12th and December 18th.
You can find a list of participating properties on the promotion website, as well as the FAQ.
Earning is straightforward: you get a multiplier based on the length of your stay and if it crosses into the weekend. One catch is that Sunday nights do not count as weekend nights; only Friday and Saturday nights will cause a 3x multiplier to be applied.