1) Head to Costco and buy a $50 Jamba Juice Gift Card for $39.99, a 20% savings. These are not available at Costco.com, unfortunately. Costco is secretly a great source of many other discounted gift cards, as well.
2) Print out this buy one get one free Jamba Juice Coupon. Hurry, it is only valid through today! It will come again, though (we've seen a half-dozen others in the past 12 months), so stock up on the gift cards for when they do.
I paid about $6 after tax for two "Power" (read: large) smoothies that should have cost me $12 but I used the gift card so the $6 really only cost me $4.80. Not too shabby!
Worth noting: check your Jamba Juice gift card balance online here or by phone at 1-866-4R-FRUIT (1-866-473-7848).