Had another thought, seem to remember a product that seemingly dispersed rain reducing the requirement to use the wipers to some degree, think it was called Rain X or similar, which might help out a tad.. Also if using just water in the washers perhaps try buying windscreen washer fluid which I suspect has a degreaser built in to assist in cutting down the oil that inevitably ends up on the screen. Also if that may not appeal, and oil/crud might be part of the issue, either vinegar or Fairy liquid might be worthwhile trying..?
