Paytronix, a leader in retail loyalty solutions, holds their own User Experience conference, and this year they asked attendees to share their most important advice for each other. From their October 2015 newsletter: 

If you missed PXUX this year, here are the key takeaways fifty of your peers identified on our last day of the event:

1. Don’t give mass discounts to high-frequency members – they will come in anyway.
2. Use target and control campaigns to calculate incremental lift and definitively answer the question: “Would they have come in anyway?”
3. When you send an email, resend the exact content a few days later with a different subject line to those who did not open the first email.