For a couple of years (I don't do it anymore because it's too time consuming), I included this note on a strip of paper in the package. I had the note saved as a Word document, 8 would fit on a sheet of paper, and I would print out a new sheet whenever I needed to. I would always use pretty colored paper so it would stand out. I got on the average about 75% of my buyers leaving feedback with the note alone.
Here's what it says:
It has been a pleasure doing business with you. I will be leaving
positive feedback on your behalf and would be delighted if you
would do the same for me. Thanks, Dyan
If you would like to send it in an email after the person received the package, it would probably be as effective.
Hope this helps!