I don't do across the board rate raises but I adjust my cost of doing business every year and then adjust the rates of the customers that need to be increased so that we maintain our profit margin.
Try something like this,
"Due to an increase in our cost to deliver the services that we are currently providing to you, we are going to have to adjust your rate slightly. Your new rate beginning next month will be $XXX. Please give me a call if you have any questions.