B&B, you might want to do a small test to see what kind of cancellation rate you are going to get when you raise your prices. Pick a few clients you would not mind losing, and send them a price increase letter. If nobody cancels, you are probably safe in going for a more general increase of about the same percentage.
once you decide you are going to go through with a price increase, you can minimize your odds of cancellations by doing them during peak season. Your customers may make an attempt to get another bid, and the guy may not even show up if it is high season. Even if he does, the bid will be higher than when people are hungry for work. In our area, April and May are just crazy.
In regard to the method, I would definitely send a letter. Make it as nice as possible, but make it clear the price increase is going to start at some specific date in the near future (we usually gave people a few weeks). Often people will do the easiest thing, which in this case is nothing. If you call them on the phone, they may reflexively cancel. Good luck.