Go here. https://www.xbox.com/en-US/live/gold
Click on give a gift and purchase 3 months. The price will be $9.99 for 3 months and renewal will show as $24.99. Most important is, it works as many times as you want. Once you purchase, click on manage and turn off recurring billing and buy 3 months again.
This may be YMMV, however then I will recommend checking the 12 months as well.
Edit: I thought this much details will be enough however I am adding more so that people can replicate it if they see this offer.
1. Go to the link above
2. Buy 1 month if it is showing $1 (Skip to step 4 otherwise)
3. Cancel recurring billing by clicking on "Manage"
4. Now go back and see 3 months
5. If this shows $24.99 price scratched and says 60% discount and price of $9.99 then you are golden
6. Buy 3 months with $9.99
7. Cancel recurring billing by clicking on "Manage"
8. Repeat step 6 and 7 as many times as you want!