In running our first introductory "fellowship" at my university I've noticed that it's hard (even after nudging) to get post-fellowship feedback. I've also started sending out anonymous feedback forms to people I talk to but no one usually gets around to filling it.
Does anyone have any useful tips on getting more people to give feedback?

I was just going to say, have everyone fill out the survey either at beginning or end of the session. I don't think you need paper slips, but participation is much higher if it is in session.
I don't agree with paying people to fill it out (because it removes intrinsic motivation, because it might seem weird to participants why this is worth paying for). I think having them fill it out in session should be sufficient to get nearly everyone to fill it out.
This works better than 'send later' because
This works really well in my experience too.