is an interesting thread involving positive feedback over in the CQ section.
There isn't really a system in place to force members to leave feedback each other. About all I can think of doing would be to go through your ads/ads you've bought from and view the users profiles and see if they have a +1 from you, and match those up against the +1's showing up in your profile.
As far as getting members to leave positive feedback, i've always had decent luck with just sending them an IM letting them know i've left a +1 for them and would appreciate it if they could do the same for me. Goodluck!