Spend 2400MS Points ($30) between 12:01a.m. ET on October 1 and 11:59p.m. ET on October 31, and Microsoft will reward you with 800MS Points ($10) by the end of November.
The "Thank You" gift promotion qualifies for all Xbox Live Marketplace video game-based content. That includes game DLC, Xbox Live Arcade titles and Avatar Marketplace swag. Renting Troll 2 six times in one month? That doesn't count, but is still a great idea.
(Ed. Note: Offer available Worldwide in all Xbox Live countries, excluding Japan, Italy, and France.)