Rain X is ok but doesn't last long at all, what you need is some Gtechniq G1 Glass Sealant, it's amazing stuff and lasts forever (well 20,000-30,000 miles) Or if you do mostly town driving, ie. lower speeds and such you can use G3 as this repels water at lower speeds but doesn't last as long. The G1 is much more durable than the G3 though and the G3 doesn't like wipers as they tend to remove it.

Another good glass sealant is Autobrite Repel This gets great reviews, especially for the repellancy at lower speeds