Bandana folded into a triangle and tied at the front around the neck. An alternative with a large enough one is to wear it the traditional way on your head, and the tails from where it is tied will help cover the neck. I adopt the latter method with most of my riding and haven't had neck sunburn, but when off the bike I've used the first method with great effect when wearing T-shirts.

In high summer, you could also look at a good quality, lightweight cotton business shirt so the collar can be turned up. Helps with the cool factor as your sweat evaporates.