If you are wanting to reduce the number of reminders you are using each month you can look at these options.
Go to Reports > Billing History and check how many reminders you are using per appointment. If this is above 1 you might want to think about editing your scripts so they fall below 160 characters.
For each appointment are you sending more than 1 reminder? If you are sending a SMS multiple times like 7 days before, 48 hours before, 1 day before. If possible you could look at reducing this.
Everytime you get a reply from a customer are you sending that reply back to your mobile? If so do need this. Can you just get an email instead? Go to Calendar > Edit Calendars > Edit and check where replies are being sent to.