If not a step stool I would first use a wash mitten ( they make microfiber and many others ) please don't use terry cloth towels on it. They can put small scratches in your clearcoat making more work for you in the future. If you can't reach with that mitten I would first try putting it on the end of a broom handle...if it doesn't stay tape it on or rubber band temporarily. Brushes are only good for areas like wheels. Even in that case it better be a very soft brush...they are also painted surfaces. Check out
http://www.autogeek.com for some of the best washing/detailing info out there.