I put my prices up every so often (Once a year- ish) but like the "do it in Spring idea!" I tend to go for a £1 - £1.50 depending on whether the work is below a tenner or above.
I use slips to get money sent and I simply write that their window prices will go up "as of and including next clean". I've never lost anyone but be prepared for "Can you make it every two months?" to which my reply is usually "Yes, but my rate will increase by 50% if I do because of the extra accumulation of dirt!" (Not if it's lead though - it's monthly or not at all - no wfp yet!)
