First step; make sure and use a good quality wash mitt, applicator, and towel.
Second step; look for a good quality wax/polish, as well as a product for removing/hiding swirl marks. Black is very unforgiving, and as hard as you may try, "blemishes" are inevitable.
I'd wash the car, and get a good coat of wax/polish on the stripes asap.
And remember, the more you "touch" the paint, the more the possibility of swirl marks, etc., increases.