How to motivate students to do readings
I require students to fill out a short summary sheet of the reading prior to the start of class for classes with small group work based on required reading. The summary sheet looks something like a structured abstract of the reading. In total it is less than 200 words of writing. I make my summary sheet available after the class. Based on feedback, the students like the sheets since they help them in reviewing for the exams. It also helps them gauge differences between what I think is important and what they think is important.

I divide the class into groups based on if the students have completed the sheet or not. Generally, I tell students to go to one side of the room if they have completed the sheets and the other side if they have not. I and the students can then quickly visually inspect the sheets to see if they are completed or empty. The groups of students who have completed the sheet get to work on the group activity, for which having the sheet is useful. The groups of students who have not completed the sheet work on the sheet and if time allows work on the activity. I make it a point to reassign students if the sheet is filled out with nonsense.