Best way to raise prices?

What is the best way to go about raising your rates on residential yards?? I have about 20-30 yards I have done for the past 3-4 years for the same $ and it's time I get a little more for them. I'm thinking all corner lots (about 12,500-15,000sq ft) to $35 from 30 and the small (4,000-6,000) to $30 from $25. How should I go about doing this?? Write a letter or call them up?? What's the best way to word a letter letting them know about a price increase??

 

"Your rate has been increased. The new rate is $ _______ ." (fill in the blank)

That's about all you have to do. If you are feeling brave, throw in a sentence or 2 about increasing costs, etc.