When I've had time to invest I've had success with writing feedback or descriptions (such as "this candidate really needs to address basic manipulation of surds") on separate cards/post-it notes and having students decide which of them receives which comment. (This is easier if you decide on a seating plan in advance and get students to work in pairs). Students have to look properly through both their own and each other's papers for evidence for each comment. I believe the students also benefit from having a mixture of positive and negative comments. Another idea is to get students to work in pairs on questions which caused most problems (again, this might be differentiated by you), with you providing hints (I've not had much success with competent students doing this, but that's an option). My favourite style for short exam questions is to do some quick fire questioning with mini white boards on similar problems, finishing with the one which caused problems in the exam. I hope these help, and sorry I can't figure out how to format my posts.