Summarize this article:

No sunscreen can guarantee you complete protection, but an SPF 100 sunscreen blocks about 99% of the sun’s harmful UVB rays. Make sure it is labeled as broad spectrum, so that it will protect you from UVA rays as well.  Look for a waterproof or sweat-proof sunscreen as well. Check to see if it will protect your neck for 40 or 80 minutes under wet conditions. For even more protection, rub on a layer of lotion sunscreen. Then, follow it with an application of sprayed sunscreen. Most people make the mistake of applying too little sunscreen for proper protection. Be generous when working the sunscreen into your skin. Use your fingers to feel across your neck to ensure that it is entirely covered. It’s generally best to apply your sunscreen at least 15 minutes before going out into the sun. This gives the sunscreen time to form a protective barrier over your skin. Sunscreen will eventually wear off and lose its effectiveness under normal conditions. If you are swimming or if you wipe your neck with a towel, then you may need to re-apply even sooner. Just to be clear, a higher SPF does not mean that it lasts longer.

Summary:
Apply a high SPF, broad spectrum sunscreen. Rub on 1 fluid ounce (30 ml) of sunscreen across your upper body, including your neck. Re-apply sunscreen to your neck every 2 hours.