I prefer using Raise.Com
I can usually get 2-4% off face value of HomeDepot cards. And I can get a bonus 1 AA mile per $ through the aadvantage shopping portal. So 2mi/$ for home depot cards (Using AA MC), then shop online at HomeDepots site via AA shopping for another 2mi/$ - so 4mi/$ and a 2-4 discount.
Sign up for the Raise email blast, they send out 4 or 5% discount codes for ALL GC's on their site, usually 1 discount like that per 4-6 weeks. It will stack with the current discounted rate on the card as well as the shopping portal.
OR if you have a Safeway near by, get 2x Fuel Rewards points on store GC purchases. The gas savings are a nice kickback on top of the miles or points.