I have been an active seller for the past 10 years and I figure if I get half the people I sell to to leave positive feedback, that is good. Even as an active seller, I have many times forgotten to leave feedback on purchases I have made. If it's really important to you to get feedback, there is now an app for that for like $5 month that you can use for a little while til you get your numbers up. Really though, it's not really that important. I would send out a reminder email once a week if you feel it's that important. Make it the same general words each time and it will look like ebay is sending it out and they may just provide feedback to stop the reminders. Cut, copy and paste your text and it will take 10 minutes a week.