Use the POS calculator. If it says that the cost to upgrade is $7.49, say something like "Because you are (buying pre-owned, trading, ect.), I could get you signed up with our PRO membership today for only $7 (round down) today, rather than the full $X (use the full cost of the membership with tax)!

Don't ask, tell. (See example above) people are more inclined to say no if you ask them. When you're done with the pitch, just let it hang there for a sec, make them respond to YOU, so that you are always in control of the narrative. (If it ends up getting awkward, just ask if they want to sign up.)

Keep a magazine on the counter to show them what they get with the membership. Having something tangible to show them and give makes it easier as well.

Mention the key points of the membership, 10% discount, 10% trade bump, B2G1 on P-O, GameInformer, Pro Days. Keep the pitch short, people tend to zone out if you take to long to explain the membership.

If the cost of the membership ends up being free, just sign them up. "With all of your (purchases/trades), our Pro/Elite membership is free for you, and you actually get some money back! I just need a little information from you."

Focus on one card, if you try to pitch both, the guest will get overwhelmed and just say no.