For practices that regularly have back-to-back patients or even a waitlist, a small deposit fee upon scheduling the appointment is not even questioned. If you're located in an affluent neighborhood, regularly conduct medical visits, or see patients with insurance, then you could require a small deposit fee anywhere from $15-$25.
Medical Exams
If your technician has to prepare for pre-tests, then that's another fair reason to require a deposit upon booking. This applies especially to diabetes or glaucoma tests. However, you'll need to make sure the patient's insurance plan doesn't prohibit deposits first.
Alternatives to Deposits
Another option is to take credit card info down and instate a hefty fee if the patient is over 15 minutes late or doesn't show at all. Add information in your patient reminders that states all reschedules will be pushed at least a month because of how full your patient schedule tends to be. These are better options if you see mainly college students or patients who might be turned off by the idea of paying a deposit.