Here's a list of Avis coupon codes and special AWD / coupon code combinations.

These tend to expire, I'll try to keep the list updated. or re-post as necessary. You can combine ("stack") these with AWD numbers. I got a standard car for 7 days for $177 at LAX in July by stacking A074800 (Citibank / Citicorp corporate AWD) with UUWA029 (free upgrade). I paid with a Citibank card and was not asked for any other proof of eligibility for the discount.

Coupon Code Discount
UUWA029 Free upgrade
UUWA055 Free upgrade ***EXPIRED***
MUWA006 $25 off 5+ rental ***EXPIRED***
TUWA045 Free rental day w/ 5+ day rental
MUWA192 Free day of GPS w/ 5+ day rental
J046300 Save up to 25% on weekly US / Canada reservations made from int’l locations
MUWA236 Save $5 on 1-day weekend rental
MUWA237 Save $10 on 2-day weekend rental
MUWA238 Save $15 on 3-day weekend rental
MUWA239 Save $20 on 4-day weekend rental
MUWA073 Save $50 on monthly rental
TUHA019 Best Western Rewards - free weekend day plus 750 bonus points
UPDA029 Free double upgrade in Australia
TPPA007 6th day free in Australia
UPPA067 Free single upgrade in Australia

Here are some special combinations (use both AWD number and coupon code):
AWD Number Coupon Code Result
A382900 UUEA010 Pay with MasterCard - free upgrade and save up to 20%
Q336600 UUCA011 Pay with Discover Card - free upgrade and save up to 25%
Q336600 TUCA013 Pay with Discover Card - free rental day and save up to 25%
A359807 MUGA048 AARP discount - save $15 on 2-day weekend rental

More information about each code (restrictions, etc.):
http://www.theflightexpert.com/avis-...upon-discount/