I've raised mine every 2 years. I never intentionally set it at that interval; it's just turned out that way, based on market value. Jacqueline, if your rates are lower than other MT's in your area, it's definitely time to raise them. You don't have to wait a certain amount of time, or until your schedule's a certain % booked.
When I raise my rates, I just put a sign up 2 months in advance that says:
New Rates Effective (insert date):
30 min $____
60 min $____
90 min $____
2 hour $____
I put it up in two places: the treatment room, right where my clients hang their clothes; and on my desk, right where people sit to write their checks. That way it's hard to miss. This also limits the number of conversations you need to have about the change, making it a matter-of-fact announcement instead. No explanations or excuses needed.
You deserve a raise! Good luck!