I have a subscription service that has a few dozen subscribers drop off every month because of expired credit cards. I was hoping to gain a little insight into some best practices for an email to prod someone to take the time to update their credit card information. I will certainly test variations over time, but I'd like to see what others have experienced with this.
Without knowing the numbers for the lifetime value of a customer, and the percentage of people that do give you new credit card information, we can only offer generic advice.
Ideally you should make sure that there is something in it for your customer. Something like giving them a month free service if they update their credit card details within the next 48 hours (you can play with the timing here). If you do that, the email is more of a "cool, free stuff" sort of thing than a boring task.
Assuming that you can't do this for whatever reason, I would follow the general pattern of a company that has tested this extensively. eBay and PayPal are good examples. eBay uses something like this:
Dear eBay customer,
This is a courtesy reminder that the credit card on file for your eBay account will soon be closed. In order to provide you with uninterrupted service, it is necessary that you update this information accordingly. To update your credit card information (very important) go to:
[Insert URL here]
It may take up to 24 hrs to process the credit card information.
To check your account status go to:
[Insert other URL here]
Thank you for your prompt attention to this matter and thank you for using eBay
Since you have the credit card information, you have the expiry date. A month or two, before their cards are about to expire, you might want to send them an email saying "Hey, We noticed that your blah blah credit card is about to expire. We wanted to remind you to renew it or change the payment method since we feel sad when you do not receive our service". Keep the tone light and friendly and give the email a personality, make it seem as if it is a person, "Hi I am Ben from Bro-Burgers, ... "