Good feedback is critical to creativity and learning, yet rare. Many people do not know how to actually provide effective feedback. There is increasing demand for quality feedback -- and thus feedback givers -- in learning and professional settings. This paper contributes empirical evidence that two interactive techniques -- reusable suggestions and adaptive guidance -- can improve feedback on creative work. We present these techniques embodied in the CritiqueKit system to help reviewers give specific, actionable, and justified feedback. Two real-world deployment studies and two controlled experiments with CritiqueKit found that adaptively-presented suggestions improve the quality of feedback from novice reviewers. Reviewers also reported that suggestions and guidance helped them describe their thoughts and reminded them to provide effective feedback.