I work part-time, about twelve hours a week. On average I open seven RCs in one shift (four hours). Here are my two main principles for opening RCs:
1) No apologies
You can say "would you like to open a Red Card today? I can save you 5% today and everyday," like a million times and it won't help you if your tone of voice and body language say "I'm so sorry to ask, but my boss requires it." Guests are smart...they can read you. If you, as an employee, don't think it's a good idea to have a RC, your guests won't either. Don't apologize for offering to save the guest money. Always believe in what you're selling and remember, you're HELPING the guest by offering them the 5% discount. You're doing them a favor letting them open RCs. If you're excited about it, and generally think it's a good idea, so will your guests.
2) Person-to-person
If you didn't know anything about RCs, would you be more likely to open one if the cashier said you should, and listed the benefits , or if your best friend explained all the benefits and said you should open one? You'd probably be more likely to open one if your friend recommended it. In that minute or so when the guest is checking out, your goal should be to stop interacting as cashier-guest, but rather person-to-person. When the guest sees you not as someone just trying to sell something, but as a person, the sales dynamic disappears and you just become two people chatting at the register. When you're person-to-person, your opinion becomes more legitimate in the guest's eyes, and they're more likely to believe in, and appreciate, the RC benefits you're explaining to them.
Hope this helps!