washing up liquid is good for stripping all the grease and muck off your paintwork but can be a bit harsh for using all the time.
maguires gold class shampoo is good
wax your windscreen (and other windows) when you do the rest of your car, helps the rain just run off and stops smearing of windscreen wiper, will also lengthen the life of your wipers as you wont need to use them as much to clear the rain.