How can I avoid heat rash in Vietnam this July?

I'm going to Vietnam for a month next weekend and I do sometimes get heat rash even in UK weather. It seems to be random, I know it can come from wearing too much clothes when it's hot but I always get it on my d�colletage area which is never covered up so I don't know how I can wear less. Would suncream prevent it at all? I've heard that it occurs if sweat glands get blocked, would sort of exfoliating the area help at all?