Stop Acid Attacks, a leading campaign against Acid Violence, issued the following advice for Britons:
1. Immediately wash affected body part of patient with plenty of fresh or saline water.
2. Don’t rinse the burn area with dirty water as it can cause severe infection.
3. Keep flushing the affected burn area with plenty of cool water,(not very cold) until the patient’s burning sensation starts fading. It may take 30-45 minutes.
4. Remove all the jewelry or clothing which had contact with acid.
5. Don’t apply any kind of cream, ointment on the affected area as it may slow the treatment procedure by doctors.
6. Rush the patient to a burn specialty hospital having isolated wards for burn patients
I used to work with hazchems in industry, so I'd love to know what their reasoning is for not using very cold water. Our emergency showers were at 5 degrees Celcius, the coldness slows down any chemical reaction. If we got contaminated we had to stay under for 15 mins, so any colder and they'd have been treating us for hypothermia.
It is worth bearing in mind that bleach and other alkalines have also been used in attacks and you won't know which has been used but water or saline will do for either. But speed is of the essence, I'd rather use water from a pond or similar than wait any length of time. Certainly anything else drinkable and reasonably neutral pH such as beer or milk would be preferable to waiting. Also, use lots, copious amounts. But having a small bottle of water with you all the time would at least allow you to get the worst of it off.
Also, you have to be careful not to make it worse by washing into the eyes, or nose or mouth so don't wash substances from the top of the head onto the face. If their clothes are contaminated and you can get them off without the skin peeling, cut them off rather than moving that clothing over other bits, especially the face. So having suitable scissors and latex or nitrile gloves would be essential.
Another thing to note is that if the victim is wearing any sort of cream (including cream based make-up), it may reduce the reaction but it may also stop it being washed off as effectively. I'll not even get into the issues of how plastic false eyelashes might react and end up with bits of melted plastic in the skin.
Best defence - move out of the way, no substitute to that really. Some people have a natural instinct to block, others have a natural instinct to move. It was seen in the west Cumbria shootings in 2009 that some people dived for cover, others instinctively put their hands or arms up to shield their face. The latter ones came off very badly. Also, close your eyes and if it hits you don't open them again until it is washed off. If your clothes are water repellent it could give some short term protection, at least more of it running off. But better to note that Gortex is resistant to most chemicals.