Rain X is good to start with
You really do need like 5x coats to get some life out of the repelling
it works well when its there, soon as it starts to wear off, its crap
My wipers as well didnt like the repellant at first, till i gave them a good clean, it was better but not 100%, made it bounce like hell!
Ziggy
Regular washing with a good shampoo like Mer repels well enough I think. I rarely apply Rain X or Rain Away but people always comment on how the water beads off the screen even when I haven't applied any for months just because of the shampoo.