iCal automatic email (or SMS) reminders for appointments

I schedule many individual appointments each week and would like to have iCal send an automatic reminder to my clients about 24-48 hours before each appointment. I have not found any way to do this in iCal and have only seen this question unanswered on old posts. If iCal can't do it is there an app that will do it? I have an iPhone 4S, iPad and MacBook Air, all with the latests updates. Thanks in advance.