This is a tough one, but a helpful technique is called “LCS,” or Likes, Concerns, Suggestions. When using this technique in a group setting, you set the ground rules for when responding to an idea.
First you have say what you like about it (even if there’s not much). The next step is to come up with 3 concerns you have AND a suggestion to address each of your concerns. This causes the feedback to begin on a positive note (encourages people) and by coupling suggestions with concerns, you begin to build on ideas.
It’s simple and effective. Give it a shot!