If you rent cars, then that's a good way to add to Delta. Delta routinely has richer car rental bonuses than AA, so even though I'm primarily with AA, I more often steer car rentals to Delta (and mostly increase my Delta miles only with car rentals lately!).

But you have to know where to look. Avis has a bonus which gives 5000 bonus miles for 4+ day rentals and 2500 bonus miles for 3 day rentals (US+ airport locations, any car class) through the end of 1999, but they've removed the promo page from the Delta website, so you have to go to the Avis forum here on FlyerTalk and find the recent thread about the Delta 5000 mile bonus and on the first page of the thread someone has a saved picture of the original web page showing the coupon codes and the terms. And everyone eho's been using it lately (including me), even after Delta rmoved the page from the web, has still been getting the bonus per the original (through Dec 31 2009, no GPS add-on required) terms using the original coupon codes.

Here's a link to that thread in the Avis forum:

http://www.flyertalk.com/forum/avis/...ill-alive.html

(Depending on how your browser feels at the moment, post #4 in that thread may show the picture inside the thread, or you may have to click on the link to see the picture. One way, or another, though, post #4 gives you the picture of the original promo page.)