You should SEE EVIDENCE of your students' learning EVERYDAY. Exit Slips are an excellent way to ensure that your students are reflecting on their learning as well as a way to keep tabs on their progress. I would get them to pick 2 to answer.
Formative Assessment Stoplight (great exit ticket idea)--I did something like this with my SMARTBoard last year. Then I could save the pages. I made it so the names disappeared behind the lights, that way I was the only one who could see it.