A little hint I learnt when I used to put black cars into shows.
Yes, the car needs to be washed in the shade, however like you that wasn't always possible. So, wash the car after sunset or very early in the morning.
With the chamois, I use a quality brand such as Meguiars. After using the chamois, soak it in a weak solution of clean water and car shampoo such as Soft Wash Gel. Rinse thoroughly in clean water and place in container.
I never had a streaking problem!
BA'1.5' Pursuit 290
Lightning Strike / Reflective Orange Stripes
'General' Dog - AP's German Shepherd and Best Mate - 02Dec1998-15Dec2003.
'Pepper' Dog - General's and My Little German Shepherd Sweetie - 1996?-02Apr2006.
'Sako' Dog - My Beautiful and Pretty German Shepherd - 2001?-23Aug2006.