Maybe some of the Best Buy techniques they taught me could help you out. Much of it is similar to Bungleau's suggestions, so my bad if it sounds like I'm just rewording what he says.
Offer it to every customer. Regardless of your personal feelings on the Mastercard, it isn't up to you to decide whether they want to buy it or not. Lots of people will sign up for it because of the $5 coupons you get just for spending money that they probably were going to spend anyways. It's hardly going to tear apart your conscience to do your job, is it? If they're going to end up in financial trouble because of the card you signed them up for, they're probably already in financial trouble.
Be excited about it. Usually customers won't be too keen on purchasing or signing up for something if the salesman seems bored by it.
Always point out how it benefits them. Obviously in this case, it would be the $5 gift cards they get A. for signing up, and B. for spending money they were probably going to spend there anyways.
Don't push it! I dunno if you're on commission or not or how the management at your store want you to present things, but I *never* push customers into buying anything. In my experience, if you do, they end up walking away from it.
Kakero: At most it will probably just bring down his marks on his reviews. The Mastercard signups don't directly bring money into the store, and don't directly impact their numbers.