We tend to review on a 2 yearly basis, mimimum rise is £1.00, stopped dealing with loose change a couple of years ago.
Do it around April time, people are in a better mood, you're giving them a better service as the weather is better, they therefore take it better.
We always give them a letter the month before telling them of the price increase, reasons why, etc and allow them to call us to cancel or discuss if necessary, very few do.