First off this will only work if you have a smartphone with NFC (Near Field Communication), if you have an iPhone you're out of luck.
T-mobile, AT&T and Verizon all block Google Wallet from running on their devices (though the Nexus 5 gets around it). They block Google Wallet because they have had their own NFC based payment system in the works and they don't want competition from Google. This system is called "ISIS Mobile Wallet" and it launched a week ago (Nov. 14th).
When a new payment system is launched, the company launching the system will often spend heavily promoting the new system; to promote ISIS, 20% cash back is offered on any purchase made from now til the end of January 2014 (up to $1000). The catch is that to get the 20% cash back you must be paying through an American Express Serve account that is linked to your ISIS account. American Express Serve accounts are free to open and close but have a $1 monthly fee. This would be $200 for nothing if it weren't for the limited number of locations that can receive NFC payments and the mild hassle to set it all up. Fortunately many of the places that do take NFC payments (like Walgreen's) sell Visa gift cards which you can then use almost anywhere (and you don't pay tax on gift cards).
So here's the process, it will probably take 30-60 minutes of your time, if 30-60 minutes is worth $200 to you then go for it, otherwise don't.
1. Stop by your T-mobile, AT&T or Verizon store to get a new ISIS-compatible SIM card.
2. Open an American Express Serve account and select not to buy the plastic card that comes with the account (this makes opening the account free).
3. Download the ISIS mobile app from the App Store on your phone. Or go here for T-mo here for AT&T and here for VZW.
4. Go to Walgreen's and buy $1000 in Visa gift cards using ISIS to pay. IMPORTANT: if you don't use the ISIS App on your smartphone to pay IT WON"T WORK, here's a a video of someone paying with ISIS, skip to 1:35.
5. When you get your 20% cash back, go back to Walgreen's and buy another $200 visa gift card.
6. Close your American Express Serve account to avoid the monthly fee. Keep the app and link it to an existing account if you like this new system, otherwise ditch the app and continue merrily on your way.
It would be nice to not have to carry a wallet everywhere, that's why I'm grandstanding for NFC based payment systems.