I just run a sort of continuous review of prices. we all have a few that stick in our minds because we know they are underpriced. To these I just say "Im sorry but I have to put your price up, from next time it will be £xx." Quite matter of fact, no long explainations. Most don't even query it, frankly there are some I wish would.
Some, I put the price up without telling them and they don't even query it ! I just start chaging them more.
When they say "how much is it again?" I often say £1 more than I know it should be and you would be amazed how many times that has worked.
If they query it I just nip back to the van and check the sheet, then say "Yes, you were right, it is what you said"
Once they have paid me at the higher price I make a note on the sheet to alter George when I get home.
I don't believe in blanket price changes. There are always some customers that need changing and some that don't. Work out a hit list and go for it.