When I pack my items I always put in a half page note that says "'Thanks for Shopping with Auntie Q to Bee!!"  If there was discussion or mail between the customer and me, I'll be sure to write a handwritten message acknowledging our conversation. Simply signing the note with a thanks is a great way to make an impression!  

When I use "peanuts" to package the item I write all over the box: "WARNING!! PEANUTS!"  All those scrapbookers out there can use their stamps to make the box you're shipping look almost like a present! After all, it is a present in a way that someone bought for themselves or their collection

Whenever I receive a package without any note it just seems so impersonal.  If your eBay business consistently sells a particular item, making return address labels that advertise your item  is a nice idea. You want the customer to enjoy his/her purchase. Packing well and just making the package attractive is one was to postive feedback!