External:
Be VERY careful that the neck area is free of any ointments or lotions while being radiated. They can act as a "bolus" and exacerbate the effects. Apply them immediately AFTER radiation each day. I used some stuff they gave me called "Radiacare" which was specifically made for radiated skin (it was mainly aloe vera and vitamin E as I recall). Everybody differs in their response to treatment. I had a mild sunburn effect -others get significant burns. I also had IMRT which limited the exposure areas a bit.

Be careful to never scratch the area. When it got itchy (right after treatment ended they gave me a steroid cream and that worked just fine - I only needed a few applications.

Internal:
NOT recommended: Acidic, spicey or salty items as well as tobacco and/or alcohol (including mouthwashes that include alcohol).

I know some here have recommended a salt containing rinse. I tried a 1% solution (standard saline solution) as recommended and my pain threshold went through the roof - so use caution. We have all heard the saying about "pouring salt on an open wound", well that's exactly what you doing if rinsing with a salt solution with mouth sores and blisters.

Recommended things:
Soft textured foods, moisten solid foods with cheese cause or gravy.

Pain Meds: Always take pain meds prior to eating. If the pain gets really bad when eating ask them to compound some "pink magic" swish & spit (Morphine, lidocaine & some optional ingredients) which will numb the mouth. Don't swallow it.