1. Keep it short.
Exit slips should take less than 5 minutes. They should be a small snapshot to provide you some information on student learning. I like to make sure my exit slips can fit on 1/3 of a page. This saves ink too!
I don't have desks, pencils, or any other supplies readily available for student use in my classroom because students sit on the carpet during music. After a few attempts at different systems, I've found the best one for me is to set up a clipboard with the exit slip and pencil ready to go.
I sort my clipboards into 6 stacks of 4(my average class is 24 students) and store them in front of the classroom so they're ready to be passed out when I have about 4-5 minutes left at the end of class. I have students in the front row take one and pass it back. Once students finish, I have them reattach the pencil and collect the clipboards. In the afternoon after school, I take the filled out exit slip out and replace it with a fresh one for the next day.
3. Stay organized!
If you're like me, you see a few hundred students a week. These tiny slips can become disorganized in your teacher bag or on your desk very quickly. At the end of every day, I make sure to gather them, paper clip the stack, and place a sticky note with the classroom teacher's name on them. If I want to take them home to grade or grade at a later date, I'll know which class's slips I have in front of me. After grading, I file them away their class's manilla folder. Be sure to have students write their classroom teacher's name and/or the date on their slip. It can help if the exit slips are accidentally mixed up!