Here is an actually commonsense tip...but unfortunately I didn't think of it before signing up:

--> Don't sign up for the double points unless you're pretty certain you're going to be spending a lot of nights in a Marriott hotel within the next couple of months. Particularly, since, as mentioned, the offer tends to repeat itself.

MY HORROR STORY:
11/97 - Sign up for this 2x points bonus
11/97 - Spend week in Fairfield Inn - 2000 points bonus
7/98 - Begin 1 year stint in full service Marriott Hotel. NO bonus since 4 months have long passed. Estimated lost bonus points - 132,000. Not made any easier by the fact that all my colleagues signed up in 7/98 and got the bonuses.