I always try to use prices increases to level out my price range. i.e. stick £2 on under-priced work, £1 on work I am happy with and 50p on over-priced work. That is probably not the best business practice but it works for us.
Usually after a price increase we lose a few customers, this is to be expected. As long as your overall value increases it matters not a jot whether you lose a couple or not.