At the beginning of each class I post 3-4 questions on the board to review the previous day's information. This also allows time for me to take attendance and set up the classroom for the days activity. I would allow about 10 min. for this activity and then have students present their solution. I did not collect the papers. There was not much accountability and some student's would purposefully come in late to avoid the work.
This semester I would like to have more accountability and put the responsibilty on each group to get the warm-ups done. I have created a group folder/team folder. Each folder will have a handout for each student with the day of each week. Every class period the group is to get the folder, work on the warm-up problems and write them down on the daily handout. They will then discuss their solutions in the group. At the end of the week the team will get points if all the team members completed the daily warm-up for the week. I am also hoping that this will address the issue of getting to class on time and attendance in general.