I use these questions to guide students into writing a proper lab summary/conclusion. This makes them keep it simple but address the pertinent parts of a lab. It has reminders about NOT retelling the story of the lab or using personal pronouns. I grade each question on a scale/rubric. The guidelines also include an example.
I have set up the rubric on a second document that is also available. It is meant to be printed on label stickers so you can peel and stick it to the lab where ever the lab summary happens to be. This has saved me many hours in grading and writing the same comments.