I have tried a few different ways of giving exit tickets.
I have used slips of paper:
I also recently started using sticky notes. I leave out a sign with the question and a few pads of sticky notes. I have two places students can leave their sticky notes - a "parking lot" and a tree. This lets me do sticky exit tickets with back to back classes.
My last type of exit ticket is electronic. I used Wallwisher last week to have 4th graders post questions for an upcoming author visit. It was not totally smooth...some kids didn't remember to click "ok" when they were done typing and their stickies didn't stay posted. But, I think with some practice, this might be a fun way to evaluate at the end of a class.