I used Swissol Zuffenhausen wax on the Z4, and plan to get a coat on the coupe in the next couple of months. You apply it with your hands, leave for 15 mins (I think) then buff off. Its hard work but really does give a very waxy look and most bits of muck/dirt just slide off (even bird poo).
Its quite expensive though - £70 for a small tub.
but I would highly recommend it.
http://www.swissol.com/E/GB/Produkte_Lack3.htm