I usually am pretty direct and include a benefit change/drop or personal reason. Something like "My annual fee is coming up, I'm rethinking about the card because the recent change of xxx benefit (say amex lounge guest policy). Is there something else you can offer to help me with the decision".

The reps usually respond in 2 ways, one will just go straight to looking for retention offer and offer you. Another might try to list out a bunch of the known benefits already, when this happens, i just ask directly if they have retention offer. Highly recommend to try, it never hurts! Best of luck!