These days, I don't give feedback until the buyer has left it for me first. I got sick of leaving everyone feedback and getting nothing in return. If I don't get it, neither do it. 

I will give a seller feedback as soon as I have received the item, so they know I have got it. If it's an international seller, I send them a message as well if the item has come untracked. Just a courtesy thing on my part.

I don't know if there's any way to encourage people to leave it. Sometimes I think I'd rather not get it at all than someone give me not so good feedback, or a raving green dot feedback and a sneaky low star rating (which has happened).