My cars just short of 30 years old and its white.
When I first bought the car i found it fairly difficult to keep clean, all the dirt stuck to it and the paintwork was really rough to the touch and dull.
I spent a weekend giving her a good once over, proper wash as stated above, removed all the sh*t with a clay bar and spent the rest of the time cutting the paint back using a DAS-6 and some meguiars.
Once I was happy with that, A good polish up and a few coats of wax.
I give it a good wash every 2/3 weeks now as I find the dirt doesnt really stick to it too much, but I like to keep ontop of it.
Every other month I'll give it a coat of wax and the paintwork still looks as if its recently rolled out of the factory