If you do not prefer buying new equipment, another thing you can do is open the door and step on the side of the car as a foot hold to reach the roof. Just a little extra work after to clean the part you stepped on which should be easy since its usually plastic.
Short answer is there isn't an easy way to do it. Roof racks, moldings etc make it difficuly. A work platform really helps vs a ladder but what ever gets you up high enough to clean polish and wax that's what you have to do.